so bought this game a while ago, and i LOVE it

unfortunately i'm doing not too great. i've gotten the hang of a couple things, but there's some stuff that i'm still trying to figure out

if anyone could help me with some of the following stuff, or anything else that someone could bring up, that'd be great

room placement (keeping imps out of libraries, optimal training room size, and the such)
keeping infighting to a minimum (because BOTH flies and spiders are great to have)
dealing with and using traps (not going to lie, just lost a game due to enemy trap placement)

Wikia actually has some good resources on the OC levels, costs and stats for the training room, and [url=http://dungeonkeeper.wikia.com/wiki/Category:Creatures]creature tables[/url] to aid your decisions and um, stuff.

To keep imps out of libraries, make sure it is not a traffic funnel. Like if they have to go through the library to get from the treasure room to the dig site, that is bad. Ideally, just 1 entrance to the room. Also make them fortify all the walls in the library early, and at once, so they don't go back in trying to do that.

Creatures that hate each other need separate lairs. They only fight if one of them is trying to use a lair and the other one spots it. they can otherwise work together.

Use traps in corridors. Make corridors specially for traps. Probably the most effective trap is a Magic Door with a boulder right behind it. The super heavy door collects enemies bashing away at it, and as soon as they finish it off, the boulder is released and they all become very flat.

The hardest trap to deal with when claiming tile is lightning. Have two imps in hand. Use one suicide imp to trigger the trap when it tries to claim the tile. Drop the second one to claim the tile when the lightning trap stops buzzing. There is a very brief window of safety.

Another trap that is dangerous as hell is a when the enemy keeper lays boulders. Just watch out for them when you invade with call to arms. Scramble to pick everyone up out of its way... nothing else you can do...

so the only issue for natural enemies is lairs? everything else (training, hatcheries, the like) is fair game?

Exactly. If you had, for example, Skeletons and Bile Demons living together in the same lair, they will attack each other. Although you can have them both work in the same workshop with no problems at all.

As for the imps getting attacked in libraries. It's been awhile so please correct me if I'm wrong, but only warlocks that have leveled up to 2 or higher will attack imps. Nothing else will. So if you put Lv.1 warlocks, or any other creature that is willing to work in the library won't attack them.

Edit: Luckily I still had a save file and I tried it with both a Mistress and a Vampire, both lv.10. Neither attacked the imps I placed in the room, and I locked them in to make sure they stayed there. It is safe to say only Warlocks, and maybe Wizards attack things in the library, leveled up to 2 or higher so they have spells to cast.

By the way, selling your library makes your spells disappear, so you need to research them again. It does go much faster though.

lvl2+ warlocks and lvl1+ wizards (if you capture one) - these are the only units that stress out in the library, and it is when they acquire a ranged attack, they will start throwing it around

-Make rooms rectangular when it is feasible. For those less math inclined, squares are a kind of rectangle, and square rooms work fine. A room needs to be at least 3x3 (so it has at least one middle tile) to be useful. More middle tiles in the room mean the room is more efficient. The graveyard is about the only exception; any graveyard tile with a gravestone is useful, and a gravestone is present if the graveyard tile is adjacent to at least two other graveyard tiles (so a 2x2 graveyard can hold some bodies).

-Put a door on the training room and lock it when you don't want creatures training. Training costs gold, and it can quickly empty your treasury if you don't have some sort of mitigation (gems, preventing training, controlling the size/pace of training).

-(Most) Creatures won't wander into a room unless that room can be used as a hallway to somewhere they want to go. Keep imps out of the library by giving them no reason to go in there (ex. all walls fortified and library is a dead end). Learn what creatures like to do, and lay out your dungeon with those interests in mind. A long walk to a treasury is better than a long walk to a hatchery, because a hatchery is visited much more frequently than a treasury.

-If your workshop is filling up (you keep hearing "Your workshop is not big enough"), then sell some doors and traps or expand the workshop. If creatures that will work are not working, fix that. A bigger workshop is better. A workshop can be used to make money even if all gold blocks in a level are mined out. -Some creatures are opposed to other creatures (ex. spiders versus flies, skeletons versus bile demons), and they'll fight if one ends up in the lair of the other. Build separate lairs for the hostile types, and be active about sorting them into their respective lairs. If you drop a monster into a lair, it will build its home in that lair if there is enough space (even if it previously was in a different lair). Most monsters won't wander into other lairs unless it happens to be the shortest path to a room they want to get to (so don't build a backup lair in the only path to your treasury or hatchery). You'll have to be active about watching your flies though, because they like to wander around and WILL wander into a spider's lair on their own. I mostly don't bother with traps, except maybe boulder traps (be careful that it doesn't kill your own minions!) and the occasional lightning trap. I just sell traps for gold.

Enemy traps are a little harder, and the tactics depend on the trap.
-An alarm trap can basically be ignored; if you want to discharge it completely, then possess a creature and walk across it until it runs out.
-You can safely trigger poison gas traps with either a Bile Demon or a Skeleton; both are immune to poison gas.
-A lightning trap is better handled with a creature that has the heal spell as an ability (ex. Demon Spawn, Dragon); trigger the trap, heal the damage, rinse and repeat. Be careful that you don't take damage too fast (killing the creature), and if necessary get a different creature to keep working on running out the trap. Tristan pointed out how you can disarm a lightning trap by claiming its tile (when you claim a tile, any enemy traps on it will be removed regardless of number of charges left); the window of time to claim the tile while the trap resets is very short, but doable. Keep in mind that a single charge will shoot ~3 bolts of lightning, so don't drop the second imp too quickly!
-A boulder trap will insta-gib pretty much anything in its path; either have a square of water/lava for it to roll into, or place a door (it will break the door and be destroyed itself; I suggest a wood door for this), or at least make sure it won't going rolling into any other of your creatures when you trigger it with a sacrificial imp. If there is space to dodge and your creature is fast enough, then you can trigger it and get out of the boulder's path. Many enemy boulders are in single space corridors, so dodging isn't usually an option.
-A word of power trap never seemed to do much damage, though it does kill low level imps. Get a creature that can heal itself (if needed) and just trigger it.
-A lava trap will turn the tile into lava. Trigger it with something immune to lava (demon spawn, dragon, hell hound, horned reaper), or just quickly walk out of the lava when the trap triggers (level 1 imps might not be fast enough). Enemies don't cross lava unless they have natural immunity, so you can use this trap to "block" sections of your dungeon.

Room sizes is one thing, but room efficiency is key. Look at the lower bar of the room flag to see room efficiency. A training room with half efficiency will train your creatures half as slow as a room with full efficiency, making a massive difference in gold. This goes for every room.

Large rooms are efficient, but neighboring tiles matter too. Reinforced walls and doors provide the highest efficiency, dirt/gold/gems/rock medium efficiency and all other tiles provide no efficiency. A 3x3 treasure room with a wall and a door can hold much, much more gold than 100 randomly placed treasure room tiles.

If you don't use keeperfx there's a bug where you can also use bridges and guardposts to provide maximum efficiency. This makes the best dungeon a dungeon without any walls and door but with guardposts between all the rooms.

Guard posts surrounding the room interfere with pathing and thru-traffic will do weird zigzags to get through. At least in vanilla. I would only encase rooms where there is no other choice, like no walls. Using them in place of doors, just having 1 tile, seems to minimize the zigzagging.