I'd start by rounding out your black-skirt tetra school by adding 3-4 more.
They're fairly nippy as far as tetras go, and they do get fairly large as far as tetras go, so I'd be wary of adding a gourami in with them in that size of tank.
One clown pleco would be plenty.
You could add 2 otos as well, or even 3 sterbai cories. If you didn't want the pleco, then I'd say do 5 cories instead.
This leaves you with roughly:
5-6 black-skirt tetras
1 clown pleco
2 otos OR 3 sterbai cories
I'd say that's pretty much fully-stocked right there.
Some might even say overstocked, but certainly do-able if you keep on top of water changes. And keep in mind that's just my suggestion as far as fish selection goes (based on what you said about your current tetras and other prospective stocking choices). You could take this in a whole different direction if you'd like and replace the 2 black-skirts with something else altogether.
Anyway, hope I'm making sense here. I'm up past my 'bedtime,' lol.
BV