If you leave boxer shrimps out of the equation, I would say you could keep 3 pairs of cleaner shrimps without trouble. They will pair off and they will pick one place they like in the tank. Mine hides under my anemone.
A guess, but that could be because you are not acclimatising long enough, they need several hours to do so, also do you test for phosphates? have you ever treated the tank with copper based treatments?