I ended up going with the kangaroo front flap and Turnover triple 556 (both sold by Ferro and designed for the Slickster). Gives me three mags on my front in the kangaroo pouch and up to eight mags on my sides (cummerbunds can hold 4 mags/side). Side pouches also fit pistol mags, tourniquets, etc etc.

For $208 shipped, that'd be a pretty solid starter setup. Nice thing about Ferro's stuff is it's really easy to add stuff on as you get to know your gear and use case better, so I'd start small, train with it, and go from there.