Damn TheBigOne beat me to it
But i'll say it anyways
1. Yeah, any combinations as answered. I personally think it'd be really cool to play a 3 Generals vs. 3 Assassins sort of match.
2. Single player will basically just be training, I believe. Which is how I like it. ;o
3. It's sort of Aeon of Stryfe or however you spell it inspired with the Assassins, and for Generals it's more like an RTS but I don't think theres a heavy base building or economy aspect.. I believe it's just making towers, buildings which build the units, and getting gold and xp from killing and maybe you get a set income of gold.
Not everything is set in stone at this point and devs don't want to release all the information right away.
Scathis, the lead dev, mentioned something about fighting elements but wouldn't go on further. Hopefully this means Assassins are very very twitch oreinted, as much or more than you'd imagine.