Mollies, and Guppies and Shrimplets, oh my!

My small, female black Molly looked like she would explode tonight, so I put her in a breeder. She had 30 babies. I put them into my grow out tanks. I also had baby guppies tonight, and when I looked in the growout tank, my Cherry Shrimp also had a litter or two. Finally they have started breeding. Too a long time to get them to breed.

The grow out tank had a beautiful grey Betta, and he has disappeared. I have no idea where he went. He was in there with algae eating shrimp, so there are no predators. Just one day he was gone. I haven't found his body, and I am bummed. Tonight, I noticed that my very small Pleco (less than 2") has his tail gone and raggedness to his fins. He is in the grow out tank too. What is happening to these fish? Algae eating shrimp and cherry shrimp are peaceful. I moved the Pleco in my hospital tank. I wonder if it is a bacteria attacking the fish. I will need to do a water change and maybe treat with an antibiotic, and be very careful not to suck up the 1/4 inch baby mollies and 1/8 cherry shrimp. Anyone local want some mollies in about two months (of course only if they are healthy)? :) (Free to a good home -- established aquarium or pond.)