I suggest you using something for a basis. For example you can use
Strategus. This is an engine for real time strategies. I think it is a good basis for a larger project. It uses SDL. Actually, this was freecraft, but it was renamed because blizzard was not pleased by this game (you can read more on
www.freecraft.org ).
I would join your team, but now I am developing other game. I use allegro and I can say that this is a good and simple library, but it is better for windows. You want a good performance with allegro under linux you have to use DGA and root privileges are required( and good graphic drives). When I saw freecraft I was amazed by its performance (1024x768 without any problems!!!) and it doesn't require to run as root (I haven't even installed nvidia drivers!!). I looked at the documentation of SDL and I saw that this is more complicated than allegro, but also it have more flexibility!!!