I had a pair of black mollies in my 29g, but the recent move over to my biocube didn't go so well for them. They were fine in the 29g and didn't bother anything and roamed around the tank at the top and bottom picking at algae for a few months.
I've had sliver mollies before in my old tank and they behaved differently than the black ones, or at least their grazing behavior did. Overall I wouldn't do it again. They don't do as well with higher current and I think that is what eventually got to my black mollies.
BTW, they don't bother any other fish, but they will compete for food when feeding the tank.