Pea puffers in a 125 heavily planted might be the coolest thing someone could ever do. Add in some bumblebee gobies and scarlet badises and you'd be set.
I'd be very interested to see this done! I think you'd be able to witness some great natural behaviours that you just can't see with them in a smaller tank.
FWIW...we started with 40+ in a densely planted 90. Ended up with 2. I don't know if it was a lack of food--we fed snails, live and frozen brine, black worms and frozen krill--or if they killed each other off. I suspect some of the former when there were lots, the latter once it dropped below about 10. Despite our best efforts to make sure every fish had fat bellies each day, numbers just kept dropping. Heartbreaking. They were awesome to watch display pack behavior hunting live food, but spent an awful lot of time brutalizing each other.
Peas are one of my fav yet-to-be-done things. A school would be great. As for reducing numbers, maybe it is indeed food, like SW chromis, where if they don't get fed continuously they kill the smaller ones off until the amount of food they get is enough for them (and, if you do feed enough continuously, any number of them will stay alive).