Snipe,
I think you've done very well to have collected 100 fry. I can tell you that I have never been able to raise any young Gambusia in my tank due to predation by the other fish. I did try to use an excluder and within hours of putting a pregnant female in the excluder there were a couple dozen fry expelled. They were still laden with yolks and probably those that were already swimming met their end by swimming out of the excluder. I hadn't expected fry being expelled that were in yolk stage ... perhaps the mother Gam was stressed ... just not sure.
I think I agree that it would be beneficial to single the species out and reduce the number for best results.
With regard to mortality, I recall reading that mortality is highly linked to spawning activity for red shiner (even BG). It's like after they pass on their genes they simply slip away and make room for the new generation. Some obviously live longer but there seems to be correlation between spawning and end of life.
