It great to read how your RSH and BNM are reproducing in your pond with an already establish population of fish. Congratulations. That's really good news!
Another surprise (to me) was how many 1/2" minnow fry a Gambusia will consume. I have 2 female skeeter fish in my big tank and even though they get more flakes than they want, they will hammer minnow fry. That alone makes me think Gams are not good to have with reproducing minnow populations.
I think Gams would prefer to eat fry over anything else. But I think this is also true of SMB, RES, and BG. I will add some thoughts on Gams. In monoculture, Gams do not attain the standing weights that other fish can, including other minnows. Beyond fertility, the standing weights they can achieve is highly dependent on the quantity of edge a BOW has as compared to the surface area. Because of this, they attain higher standing weights per unit area in smaller ponds than in larger ones. So they do have a niche along the edges which in part limits them.
Gams also limit themselves. After they reach a critical density, recruitment falls way off. This is probably due at least in part to fry predation. So gams will add more forage if they are being cropped by predators. The interesting thing about Gams is how much forage can produce in relation to their maximum attainable standing weight. They can produce many times this maximum standing weight in forage annually under cropping. The contribution of Gams is so good that I have to say it is better to have Gams than no minnows at all.
Given that Gams fill a niche, I think whether they will be a net positive in a pond with other minnows is an "it depends" that depends in part on what the other minnows are and what niches they fill as preferred habitat. To be sure, their niches will overlap. For example RSH will utilize the edges that the Gams prefer. It may be possible that the edges can support a greater weight/unit area of Gams than they can of RSH. Given that RSH prefer midwater pelagic water, fry predation by Gams may be low enough that the Gams make up for that negative. I don't know and I am reluctant to guess but I will say this. Prey diversity is best when they are filling underutilized niches in time, space, and primary food sources. So diversity in peak reproduction, preferred habitat, and types of preferred foods should maximize forage production. From this perspective, the combination you are working with seems almost ideal from the minnow perspective. RSH for midwater, BNM for benthic water, and Gams for the edges.
Even so, as long as the BNM and RSH are reproducing in good numbers ... you may not miss the Gams.