You can switch teams, but I think it is still a good idea to all be on team green, if only because other people usually draw alliances according to teams, so if you are on another team, people from green team won't usually be as friendly, so it could be a conflict of interests.
Tomorrow, Ent, I should be able to give you a decent boost, somewhere along the lines of 750 Marks, and more, as time goes on. I would give you more, but I don't want to fall too far behind on upgrading my own country. Also, I should be able to help you militarily, if someone attacks you. Hopefully, we can get you to a reasonable level, and then we can both help anyone else who wants to join.
Alliances aren't really officially supported, i.e. there is no game feature that forms an alliance. It is just an agreement between you and I that we will support each other militarily, financially, whatever. If you want a more official feel, the more established alliances will put something in their profile to signify that they are part of an established alliance.