I have added some before, as long as they are about the same size, and you have enough room in the tank, there shouldn't be a problem. There is always the chance that there can be death w/ p's.
I don't think I would add anymore. I have 4 in a 75 (am looking at a 125) they do alright in there but there is alittle aggro once in a while. If I remember right (which is unlikely) I think it is like 20 gal per p. Although if you are planning on upgrading soon you could add some depending on the size of tank you plan on upgrading to.
I hope this helps soom.