For 3s, I think the multiboxer should be required to play all the toons.
For 5s, you could have two brackets, one where a multiboxer must control 4 toons and one where they must control all 5. I plan on playing both buckets, and would be fine if it was one bucket for either 4 or 5 toons controlled, but I don't think you want to limit it to teams who control all 5 toons.

I'd be careful with RBGs as there are active boxers who clearly win trade. I suppose that could be the same for arena, which is why I support video/stream verification. Certainly not all the matches, but if someone is 2k then I'd like to at least seem them beat a couple 1700 teams on a stream or in a video. I believe streaming/videos build the community more than any other method, including this forum, and encouraging the behavior makes everything more interesting!

I disagree with prizes for individual classes, solo toons, or group compositions.