You can easily have as many as ten shrimp per gallon, but I would start with half that and just let them breed, in the case of cherries. Has anyone here had success breeding amanos? I'm trying crystal reds at the moment.
Amanos are extremely hard to raise. The breeding part is easy, but the eggs must hatch in salt water. The babies aren't shrimpets when they hatch, but larva. As the weeks progress you have to slowly decrease the salinity of the water as they grow, until they are juvie amanos in freshwater. If you convert them too fast, they pretty much die immediately.
Well, the amano didn't make it, and PK got to nibble on an expensive snack. Some bettas aren't meant to live with shrimp.
Sorry to here about your amano :/ . Normally once they're full grown they're pretty betta-proof (I had a female amano once as big as my plakat.. she was actually agressive to him
so I traded her in).
Best bets with shrimp are either species separate tanks (I like 5 gals with moss, branches, IAL and sponge filters) or tons of hiding spots in a tank with fish. Shrimp friendly fish in my experience are cories, pitbull plecos, threadfin rainbows (they have extremely small mouths), and pure endlers. I've heard micro rasboras like chillis can be o-k too. Everyone else will most likely view your shrimp as a food source.
Oh, and if you want your shrimp to breed like crazy? Earthworm flake and sprulina. That stuff is like candy to them. I've had tigers steal the earthworm flake from fish. I feed other things but those two are by far the favorites (for my shrimp and my fish actually).