I've kept them with both harlequins and coral banded types (I keep the more unusual Stenopus like pyrosonotus and spinnosus). The harlequin doesn't bother anything except stars, the coral banded eat them if they can catch them, but they are tiny so in your 30 they may be able to stay clear.
As far as multiple harlwquins, IME only if purchased as a pair. I don't know for sure, but I've also never tried to add them together unless they were already paired up. I've collected them and everytime I've seen them in the wild they were paired up as well.