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What kind of tank are they going in to? Are you looking for help with algae control in addition to adding some life to the bottom of the tank?
Mixing them would be fine... but 3 RCS will disappear pretty quickly even in a small tank. If you post the size of the tank, we could make some recommendations on a decent size colony to get you started.
If you want both CRS and amanos, yes, I would say that would be good. You could go 3 & 10 I suppose, though I think 2 is a better #. Amanos get rather large. Both would be fine with the cardinals, though you will find a lot of folks who would disagree with putting cardinals in a 6 gal tank.