Those water parameters are perfect for soft slightly acidic water fish, but less so for livebearers so you will have more success with the former. With the gourami, a shoal (group) of rasbora would be perfect. Check out the fish profile for the gourami, you'll see there are issues with the Dwarf Gourami
so the Honey Gourami
is a better choice. And any of the rasbora make good tankmates. Though a 15g is small space, so be careful what you select. The Sparkling Gourami
would do well in a 15g in a small group, and rasbora. There are several species of the latter in the profiles.
For a filter, nothing beats a simple sponge filter in this sort of aquascape; none of the fish I've mentioned tolerate water currents, and with plants you don't need much of any filtration anyway, so a perfect match.
If you're interested, I just set up a 33g tank along the lines of the above, I have Sparkling and Chocolate Gourami
, rasbora and dwarf loaches. Sponge filter, minimal water movement, lots of plants (just starting out but they will fill in). Photos are under "Aquariums" below my name on the left, it is the 33g SE Asian Pond tank.