male gouramis, and even females in some species, can be extremely aggressive with other gouramis in the tank, especially males. dwarf gourami males will fight in a tank, constantly chasing and fighting.
pearl gouramis are one of the more peacful gouramis, and more than one male can be kept in a tank. they ARE gouramis though, and therfore to be safe the recommended male:female ratio is 1:3.