I mean, you've fought those boss guys and you've met those otherwise cool non-party members along your travels, and you've seen how they fight, that should give you a rather barebones idea of how those characters go about their fighting or casting or whatever techniques. And YES, you simply can't KNOW how a villain or non-party character would fare in a team battle situation with the main characters of the same game since their strengths/weaknesses were rather limited to what we only saw. And YES, this is just an IDEA. I'm not expecting it anytime soon, given the hell load of work to make this team melee as balanced as possible.

Life is short.
Let's all grab a BEER~~~!!!