[@TheLoneBanette]
Yeah. As soon as I saw Steven Universe I was sold.
Anyway, answers. I think it would be most interesting to allow people to play whichever origin they preferred, whether Homeworld, Kindergarten, or Crystal Gem. Either way it should probably focus on on the earlier beginning of the rebellion, so start just slightly before it. This would allow the players to take part in the rebellion from it's inception. As for style I think Casual would allow to keep the tone a bit more light hearted like the show and still keep character development prominent.
Also I would indeed have time to Co-GM. It would in fact be a pleasure.