Before you get anything else, you should get 3 to 6 more hatchets and 4 more kuhlis. The hatchets will live much longer if you have a school of 6 or more, and since they can be tricky, you will want to have enough so that a couple of losses won't drop the school below 6. Kuhlis tend to be much bolder and more visible in schools of 6 or more, but it's not actually a health issue like it can be with hatchets. You should stick with black kuhlis unless you get a school of 6 black and one of 6 gold. In that case they'll probably mix from time to time.
If it were me, I'd probably double the size of my neon school as well. 6 is a minimum.
ADFs may (may not too) be able to eat small neons, so bear that in mind.
A centerpiece fish is kind of tricky here. With somewhat skittish fish top, center and bottom, you'll need to find something that's not going to harass or eat them.
I might go with a pearl gourami (or two if you're heavily planted). I'd stay away from mollies. The rest of the fish in your tank are softwater - no salt at all, and mollies do best in hard water with a little salt.
Actually, if you went with 8 Hatchets, 6 black kuhlis, 15 neons, and 6 bleeding heart, pristella, or lemon tetras, that'd make a really nice tank.