I did struggle to see how the PPG could all be one character, but I'm glad they came up with something that doesn't feel too clunky. I think the PPG's ability to fly helped with that.
Usually, I only support characters fighting together as one if one of the characters doesn't have enough moveset potential on their own, and/or the characters can reasonably be placed near each other without it feeling awkward. Like, I'd be upset if Mordecai & Rigby were placed together, because it'd feel awkward, and cause there's enough stuff in Regular Show to give them each a moveset. But I think Hi-Five Ghost should assist Muscle Man, cause he can easily float beside Muscle Man and fly after him when he gets hit.