I've done huge games with both D&D and GURPS and it's worked ok. I've been running this game with GURPS, which is my preferred system. I have a lot of experience with GURPS, so I can tweak it to keep things moving, but as we've hit the 9-player mark, I've begun to wonder if there's not something out there that is better designed for dividing spotlight time into that many slices.I DMd 8 players in a 5e game but combat did not a bit of tweaking. I mostly had a pole with clothespins that tracked initiative and had them roll damage against themselves while I moved on. Going around to the player turns did slow some.
Honestly, a game with a funnel -- DCC or Shadowdark are both good choices here -- is probably the ideal way to start off running games for this group. Everyone gets to jump in, there's lots of chaos and carnage baked in, and when some people flake on future sessions -- always the way to bet -- you can just say the others fled back home to become gongfarmers.With a group that large, in that short of a time, that's going to be a tall order. When you say character development, I'm not sure if you mean personality or mechanics, so I'll answer both:
Troika, Americana, Kids on Bikes
DCC RPG - it's got enough levers and growth opportunities, but runs fast enough that people won't be waiting forever for their turn to come around in combat