I've got a 10 gallon with a healthy colony of RCS and 10 harlequin rasboras. The rasboras will eat smaller shrimplets that are 1-2mm in size if given the chance, but any bigger than that and they leave them alone. Given the size of your tank and with enough places to hide (like a carpet of plants or some dense stem plants), you can establish and maintain a healthy colony. I usually sell 15-20 sub-adult to adult sized shrimp from my colony once a month, and I started with only 11 (added the rasboras a couple of weeks after the shrimp). In my tank, adult RCS freely swim in the water column, sometimes swimming right through the rasboras' shoal with no issue.