Depending on the types of gouramis, tetras and catfish there are many choices. I hate to tell you, but at some point the Bala Sharks may out grow the tank. Fire eels are cool, but in a well decorated tank you won't see them very often. Most killie fish are top water and with the sharks that could be a problem. The Golden Wonder Killie would work. They get about 4" and can old their own. They will eat smaller fish so depending on the tetras size they would work.
I'm not sure what type of gouramis you have or how long you have had them, but most gouramis can get large if allowed to do so. If the fish are young they will grow larger than typical. I'm a fool for Leeri-Pearl Gouramis. A pair of Badis would look nice and give some shape breakup among the fish.
For the middle colunm, you can either go with a shoal of tetras or break up the fish types with barbs and livebearers. For livebearers I would go with one of the larger Goodieds. For barbs, Odessa barbs from Selectaquatics.com would blow the color off the map.
Bottom dwellers, depending on the type of cat fish you have a large group or two distinct different groups of corys would look nice. Say Jullei and Pandas. I would get at least six each.
If you let us know what you have now-gouramis, catfish, tetras, we can help you out some more.