Bahamut wouldn't need to be sized down to 1 square pixel. He could just have a completely different sprite made of him, like around the size of the regular dragons. Some summons could even be human-sized.

You're right, though. It would make the game too cheap and easy. That's why I never used aeons in FFX (except as shields), because they were so cheap. If one aeon's cheap, imagine a party of summoners, and each one is controlling a summon with the cheapness of an aeon, or even Orlandu. I guess they'd have to put some restrictions on the summons' power, or the number that could be out at one time.