Unless they can find their way through a foam filter and back up the return(through the pump impeller no less) into the main tank, I probably don't have to worry about infestation. The more the merrier; If they have the resources to produce, I want more. If not, I want less.
Sounds like MTS are the best bet. Probably about 4-6" of sand. Isolated from the rest of the tank's inhabitants.