I've got RCS from 3 separate sources. They are pretty diverse.Endlers have significantly smaller mouths than rams and angels I've seen my endlers nip at RCS, but never actually try and eat them. I wouldn't be surprised if they ate shrimplets, but I have enough plants and hiding spots for them. (Your RCS are doing very well btw; lots of babies and everyone is super bright red )
Personally I think the lower grade CRS look better, since they have a more balanced amount of red and white. I like the grade S ones.i have both rcs and crs and they do not inter breed. i suggest you go for the 5 gallon (remember the bigger the tank the larger the colony). i have a 25 gallon that is lightly planted and it has been almost 2 months sense i started it and am just now starting to count little tiny shrimplets. i would start with just cherries and when you feel you have enough experiense get some lower grade crystals. the only reason i have crystal is because i guy offered 30 for 20 buckaroos just because i am joung and have much enthusiasim about my animals (helps that i live in the same area that is grandma did). the 5 gallon should be good but if you want to have a large colony (litterally hundreds) then woud get a bigger tank. snails and shrimp go fine together. also like the others said, don't keep fish with them, even ones that have really tiny mouth as they will eat the shrimplets and you will never get an extablished colony in the normal amount of time.
good luck and may your future and current tank prospure,