For sanity's sake, I think I'll limit Sky Hobo to at most six "character slots" that he can freely switch between and that will be retained permanently. If you want to add a new persona after all the slots are filled, you need to either kill a character off or put them on a long bus trip (since it's an airborne city, it would actually have to be a long airship voyage). It doesn't matter what it is exactly, it just has to be something that ensures that no one will try to find out what happened to the character.
In addition to permanent characters, Sky Hobo can use his extraordinary disguise ability to inhabit any number of temporary characters. Temporary characters can last as long as Sky Hobo is portraying them, but as soon as he switches to a different character, they disappear forever. If a temporary character gains fame, he must be "killed off" like a character being taken off a roster before Sky Hobo can switch to another character. Alternatively, you can promote a temporary character to a permanent character if you "kill off" one of the permanent characters.
As for the performers, while they cannot inhabit one of Sky Hobo's personas themselves, they can play a side character to a persona, such as a sidekick, relative, lover, or nemesis. These side characters must always have some sort of relationship with at least one of Sky Hobo's permanent personas and will be listed in their character sheet.