I played a game on "average" but noticed there was very little combat involved. I did have a diplomat leader, but essentially nobody ever attacked me even though i had zero defending ships at all.

Antarans mostly ignored me or got wiped out. Antaran fleets are very good against two types of fleets in particular....fighter or missle heavy fleets, as spatial compressor instant kills missles and fighters. If you have decent beam tech though, they really arent that bad...

By late game the map looked like a mix of 2 AIs having roughly equal territory and 2 smaller AIs with the left overs. I never had to fight a single non-antaran fleet because everyone loved me. I had trade/research pacts with everyone(till they got killed off) and all i have to do to avoid getting pulled into a war is to decline alliance offers. AI seems particularly bad at fighting wars...their fleets just dont seem to do much. Psilon has all the techs by now and really should be steamrolling everyone but its just not happening.

Somehow this doesnt seem "intended"...all im doing is hitting end turn but the game was won a long time ago.

The previous game, i did a spy heavy approach and took uncreative/-1 research. This didnt work too well because...it seems spies arent really that effective. I had max spies on Psilon and a very high spy bonus...it still took quite a few turns to steal a single tech, and sabotage wasnt that effective. And everyone just kept declaring on me because i was stealing tech from them.

Were you playing as a charismatic race? (Humans are the only one of the default races with this trait)

One thing you could do is increase the density of races. Play on a smaller map with 8 races and you're a lot more likely to see some combat. Playing on higher difficulties also makes the AI more aggressive.

I played on a large map with 8 races. No charismatic.

hmmm ... I don't know that's odd. Usually the AI is more aggressive than that especially when they perceive weakness on your part. Antaran attacks are *usually* against the most powerful faction and early game can be a major headache especially if they attack while your fleet is out of position. Later game with the appropriate defensive tech (jumpgates and stargates in particular) they can be defeated relatively easily by virtue of you being able to marshall your forces quickly to respond. While their fleets do get bigger over time, they are never as big as the fleets you can send and you can overwhelm their tech advantage.

What is the antaran tech advantage anyway? Do they have access to tech you can never research by yourself? I managed to get some of their tech from defeating the guardian...

For fleets, is there any reason to build the smaller ships other than cost? For example, you can build 2 frigates for every destroyer, but a destroyer has more than 2x the space of a frigate...it seems bigger is ALWAYS better.

As for fleets, so far one fleet is usually enough for me as long as i keep up with drive techs. Im not sure whether thats normal. The AI doesnt seem to do "multi attacks".

That's odd. I've found usually the Antarans only attack the winning player, but maybe it's random. Their armor is good and so is their maneuverability - some of their tech can be found on Orion (if you have one leader slot available when you defeat the guardian you get a pretty powerful leader). Other tech can be gotten from capturing Antaran vessels.

Smaller vessels can be useful for cheap, quick builds if you need to replace lost units quickly. They also are more maneuverable with lower hit percentages. However, by the time you get up to Doom Stars, battleships become your "small" vessels. :) So how small you want to build really depends on your tech level and industrial capacity.

The AI can do multi-pronged attacks, but it is more rare than not - it happens much more if you fight against an allied pair. Then by fiat you will face a two front war.

What are some weapons that will work against antarans early game then?

Interceptors dont do enough damage, heavy fighters get instant killed by spatial compressor....

Missles are easily shot down and are hard to fit onto destroyer chassis in large numbers.

There's not really a lot of tech that can help you early game simply because you are so outclassed in the tech department. Bombers + assault shuttles (or troop pods) if they can get in close can be helpful - the former for damage, the latter for capture (only good early game if your race has max infantry bonuses + has kept up infantry tech). Heavy fighters come in mid-late game. Mostly you win early encounters by sheer force of numbers - have many more ships than them (since early game the fleets the Antarans send are smaller) and expect to lose a lot of them even so. Thus you sometimes have to balance how important the colony is compared to how important the fleet is.

Btw is there any way to upgrade ships to a bigger ship class, keeping all the experience, or scrap them?

You can scrap ships, but you cannot transfer a crew to a new ship - that would be nice, but is not a feature - or refit a ship to a larger hull.

How do you scrap ships then?

Edit : Nevermind, found it in the fleet menue.

Noticed something really odd. I managed to board and capture a starbase...but instead of getting a free starbase, it just dissapeared. Where did it go?

Is there any way to invade a planet with armor? Transport ships only seem to carry marines, and i dont understand the point of armor garrisons. The AI always bombards the planet till everyone is dead or almost no defenders are left anyway...

If assimilating conquered species, do they use your racial bonus(selected at game start) or theirs?

How does spying work? If you have 10 spies, and they have 10 spies, same stats...how long will it take to steal something?

Do leaders come from the same pool? E.G. Can two playersd hire the same leader with same stats but different names or are they mutually exclusive?

Why is it that sometimes a AI is missing one of your techs, and you are missing one of theirs, but they refuse to trade for it (exchange techs is grayed out)?

Do you get tech from destroying enemy ships/capturing their worlds/etc?

Transports can only carry marines, indeed. Armor, mechs, etc. simply offer another layer of protection against invading marines. While the AI may not bother to invade a planet before nuking it from orbit, multiplayer might be a different story.

I'm not sure how the outcome is calculated. There are a lot of factors involved, and luck plays a relevant role in it.
Regardless of the mechanics, one thing is for sure, spying efficency drops severely in mid-late game, even if you have max spying techs and racial bonuses. Wich is a shame, because on paper it's an awesome option.

As far as I know each leader is unique, if you hire one, nobody else can. I'm not sure what happens when you fire one though, meaning if he/she goes looking for work at another civilization or is gone forever. I've had it happen too, and I'm still wondering if it's a regular game mechanic (i.e. the civilization simply doesn't want to trade with you at the moment) or a bug.
One trick to get around it was to contact the ambassador again and again in the same turn, the option to trade would show up eventually. At least that was with an older version of the game, not sure if it's the same for the GoG version.
Also, on higher level of difficulty the AI races are less inclined to trade techs with you by default, while they trade very easily with each others. You can get tech from conquering planets (it is a random chance) or scrapping enemy ships you have captured. Destroying something doesn't yeld anything.

Avogadro answered all of your other questions pretty well so I'll just add a touch. Captured starbases can only be used during that battle - afterwards they are considered destroyed. I'm not sure if this was a deliberate balance/design decision or if permanent changing of control of starbases was something they just didn't bother implementing.

I've had AI actually invade the planet with defenders left and sometimes having the armor brigades is nice then. Of course that only buys you a turn because afterwards they bombard the hell out of it if their invasion fails. :)

I often, though not always, like to play as a telepathic race - it means ground combat is never needed to capture a planet - you can use mind control if you have I believe at least a cruiser in orbit - so I never have to build troop transports. However it costs a lot of species character points to get.

If I might add my two cents;

When capturing a planet, any and all purely military installations (star bases, ground-to-space weaponry, and ground force buildings) are destroyed, no matter how you took control of the colony. Naturally, the Missile Base, Fighter Garrison, Ground Batteries and so forth need to be eliminated just to get to the point of capturing a colony, but an orbital defense platform can be overtaken and controlled (and used to bombard the planet during combat, but not afterwards) and with a Telepathic race, you never actually have to deal with ground forces. Despite these realities, though, all orbital defences and all ground force buildings are removed from a captured colony no matter how you get ahold of it.
I assume that this is for game balance, to force you to defend a newly taken colony (i.e. not move your fleet off it immediately) if you want to keep it. Otherwise a Mind Control takeover would leave a colony with its full ground forces, potentially making it impossible to retake - or immediately giving the captor a set of marines to build troop transports for and to use against a telepathically protected enemy.

As far as leaders go, all leaders come from the same pool, and if dismissed return to it. If they're killed, though (a combat leader can be killed on ship without survival pods or without a rescuing vessel; a colony leader is killed via orbital bombardment) they're removed from the pool.