I have never kept P. guamensis, but I have had other Pomachromis species. They can be a bit territorial. Moreso than Chrysiptera damsel species, but not as bad as meaner damsels.
I'm down to one of these guys after having started with a trio three months ago. They have been model citizens thus far in my 100g community reef. Having a school of these guys would be awesome. Does anybody know where I can place a special order these fish?