Given the size of the pond (900 sq. ft.) I would also be hesitant to take up biomass by adding another large species. With a breeding population of mixed-sex BG in place, that leaves the options of 1) starting over to go same sex BG and 2) harvest, harvest, harvest!
Given no larger predator, I would consider adding those FHM (and/or Gambusia) to try and turn a non-BG eco-niche into food for BG. Larger BG should be able to utilize smaller minnows, perhaps biasing the pond in favor of the larger BG sizes a little more. But with breeders in place, you're going to have to pull out pretty much all the smaller BG you can. And all the larger females as well.