Hey man. You don't have to join leagues to play online don't worry, you can just play for fun against your brother or any of the gens players really as most people are friendly enough to exi. I would recommend joining spring classic when it rolls around in a month or so just because it's a great experience and is really fun, but you don't have to if you don't want to. Add me on aim and lets play sometime.
As for your second question, I'm not 100% sure but I don't think that you can play just two human teammates against the computer. I think you can't because playing on a team requires the controllers to be plugged into slots 1 and 3, and unless there is at least 3 players in an online game than only the first 2 controller slots are used without multitap functionality.