I have two 40 gallon over the edge filters in the tank
If "over the edge" means hang-on-the-back, one will be more than enough. In my 70g 4-foot tank, the canister gave out so I got a Fluval Quiet Flow internal filter rated for a 40g tank. All I wanted (and all you want here) is water movement with some surface disturbance at one end, and this filter delivered it. It has an internal "sponge" sort of cartridge that has activated carbon built in, but after a few weeks the carbon is not active and I just rinse it out under the tap every water change. I am assuming plants, and floating plants, here, given the fish species intended, so filtration can/should be minimal. Actually nothing more than moving water around gently, and through media to keep it clear. The plants will handle the clean aspect, along with substantial regular water changes. Hatchetfish enter this issue too.
On earlier posts, I agree with most of the fish suggestions. Skirt (aka widow) tetras are rambunctious for all the other fish and won't work. Especially with hatchetfish. Rummys need larger groups, here I would aim for 20. Otos I would stay with the six you have. Embers are fine, say 12-15; in larger tanks more are better for such small fish.
Hatchetfish...which species? This makes a difference. The smaller-sized species in the genus
Carnegiella should have larger groups, at least 20; one species or mixed (within the genus). These fish will remain at the surface permanently, unless something startles them. They rarely descend; mine sometimes did during the pitch black darkness of night. I had reason to go in the fish room a couple times with a flashlight (never turn on room lights in darkness) and was interested to frequently see the marble hatchetfish mid-level in the water.
The other larger-sized species in
Thoracocharax or
Gasteropelecus are more often below the surface and do tend to be more "playful" active. The Platinum (
Thortacocharax stellatus) especially. A group of 9-10 of one of these would be fine.
Given the other fish, any of the hatchets would work, but perhaps
Carnegiella species better.
C. strigata (marble),
C. marthae (black-winged) are the more robust;
C. myersi (pygmy) is the smallest known hatchetfish and very delicate.
You have to be careful with names of the hatchets; I have seen them inaccurate a lot, not just the common names but the scientific too.
The Harlequin seem to have left the list...if you want a rasbora, one of the related species
Trigonostigma hengeeli or
T. espei would work very well in this set-up.
I already have 6 otos in the tank, and the plants are arriving soon 
