I'd say its possible to have some apistos, convicts and firemouths. It is indeed a 150gal. Don't pack them in and i don't think you'd have a huge problem. Firemouths don't tend to be trouble makers anyway, unless picked on. Live, let live attitude. You convicts would prob. be your biggest problem as far as aggression.
What about rams, apistos, and maybe a few firemouths. Maybe some larger tetras? A couple fancy plecos would be cool also.
Or, if you want large schools of neons (i reccommend cardinals myself, they seem hardier and show more color, then just shoot for Rams, Apistos, and the tetras. And maybe a couple of fancy plecos. A large tank such as this would look awesome with lots of cardinals schooling around. Plus some Apistos poping in and out towards the bottem, flashing their colors. You could shoot for a few pairs of apistos.