How big is the tank?
I have black skirt tetras with honey dwarf gouramis with other fish in a community tank and they get along fairly well. I have a 4ft tank, so there's lots of swimming room and I have lots of plants and rocks, so there's lots of places for the to hide from each other. I've even found that the male gouramis chase the black skirt tetras which are twice as big.
Maybe it's a territorial thing. If the black skirt tetra's were there first, then by adding the gourami's the tetra might be trying to protect their territory.
I do find in my tank that the black skirts seem to stay in the same area all the time, only venturing out for feeding.