P Acei are amongst the least aggressive mbuna. At the same time, they are one of the most active mbuna as well - as you mentioned already, they like to be hanging in groups, and they are one of the few mbuna species that don't claim rock territories so they are compatible with many species. Often, they are even used as "dither fish" in mbuna tanks because they like to hang outside of the rock structure al the time, and occupies upper half of the tank more frequently than most others do. When they are around, other fishes seem to feel more comfortable coming out as well.
Just about the only thing you need to watch out for, is that they do grow up to 5 inches and they do like to wonder around alot, so watch out for their swim space...