I doubt most store staff would know the difference in gender. I acquired six of these, and lucked out with 2 male and 4 female, but until they began interacting I could not tell, they were small. Like all gourami, males are territorial so make absolutely certain you acquire the entire intended number at the same time.
You need not worry about aggression, it is normal and should play out very well. They must have a good cover of floating plants and plants near the surface, and you certainly have that. Do not have surface fish in with them though, like hatchetfish. The male gourami guard the nest of eggs and they will nip any fish getting close. I had rasboras and a group of dwarf loaches in with mine and they lived together for years. They will spawn regularly.
The males will as
@WhistlingBadger said communicate with clicks that are loud enough to be clearly heard. This species,
Tricopsis pumila, is related to the "Croaking Gourami"
Trichopsis vitatta. In a 1992 study, Friedrich Ladich et. al. determined that this sound is important not only in breeding displays but also in antagonistic displays of males to establish dominance. The "croak" is a series of double pulses generated by rapid beating of the pectoral fins. Another study by Ladich in 1998 established that the sounds vary according to the individual fish and the sound frequency had a role in determining the outcome of the encounter between rival fish.
