Actually you don't want to keep red and yellow together, at least if you are talking about neocardinia species. They will inbreed and give you ugly brownish offspring. That's my biggest peeve about keeping shrimp. If you want to have the nice colourful ones you see in the store, you have to keep each colour separate!
To start off with cherries, I'd suggest six, 4 females, two males. With proper conditions all four will be waddling with eggs in their bellies soon enough!
Sexing cherries is fairly simple. Females are larger, brighter, and have a rounded underbelly. Sometimes they are even carrying eggs. If you see that, choose that one. Males are smaller, slightly duller, and have a flat underbelly.
This site has loads of information on shrimp, from reproduction, basic needs, to what can be kept together. Even on shipping. They also have great photos for helping sex shrimp.
Assuming you go with a filter, either set up a sponge filter, or get a pre filter sponge to put over the intake, or you -will- loose your shrimp into the filter.
While this thread started as being about bettas and shrimp, there is still lots of great info about shrimp themselves! http://www.bettafish.com/showthread.php?t=119766