1) No I don't think you made a mistake. I used to love all three of them
2) For the first two games:
You can raise land with the left mouse and lower it with the right. Use this to make as much flat land for your followers as you can. Depending on the rules of the map, you might only be able to do this near your settlements, you might only be able to raise land, or only be able to lower it. You don't have much control over them, but if you build flat land they will settle on it.
As your population grows you can unleash various disasters upon the infidels in the other corner as your mana builds up. In the second game there are a lot of effects and they are neatly organised on the left side of the play area. To unleash a column of flame for example, you select the fire sub-menu and the column of fire which is the first fire spell. The first game has the buttons all over the place but you can tell which ones are disasters because there is a mana bar with the same icons.
Both games have four different follower modes they can settle, fight, band together or go to the papal magnet. The only one that makes a huge difference is go to papal magnet. In this mode, all wandering followers will go to the leader if there is one, or towards the papal magnet if there isn't. The leader will go to the papal magnet. There is an effect which moves the papal magnet, but you can only do this if you have a leader. And if your leader dies, your papal magnet is moved to where he died. On the flipside, if the enemy leader dies on your turf, you can build a wall around his magnet, throw down conversion pools and swamps and generally make it near impossible for him to get it back.
The first game has one effect and the second has six effects that turn the leader into a hero who then crusades into enemy territory and cleanses it of infidels. The papal magnet will be dropped where the hero was created. It is a good idea to build up your leader by using papal magnet mode for a while and maybe uprooting a few castles. Heroes in the second game are mostly immune to effects of the same group eg achillles is immune to fire effects.
The third game, IMO, has a decent enough tutorial.
3) The third game has Populous Reincarnated
http://popre.net/
The first two games aren't so lucky, although they are both on gamefaqs. You might want to watch a let's play or two on youtube
4) There isn't really any overarching story or anything that makes it important to play in order. I'd also say that the second game is better than the first in pretty much every way, so the first game isn't really worth playing. The third game is very different though so I suggest playing Populous 2 and Populous the Beginning in any order you like.
