I personally wouldn't do it. Tiger barbs (mine any way) eat all the food before it even gets to the midwater so I do not think something as small as a desert goby or peacock Gudgeon could easily get food. You may be able to pull it off if you make sure that they get properly fed. Also the shark may prove to be an issue. My rainbow, when it's feeding time, goes down to the bottom and sucks up all the food real fast, especially pellets. That plus tiger barbs and your other assortment of fish may prove extremely difficult to feed them. Also also since gobies and Gudgeons are bottom dwellers you may have some issues with territory with your shark. If the shark is a red tail he will likely find them as a very easy to get target, and it wouldn't surprise me if he ended up killing them. If it's a rainbow shark he'll just likely defend his territory, now to most "big" fish when a rainbow chases em off they just swim away and get back to their school or whatever they were up to, but to a small fish like a Gudgeon that would be absolutely terrifying and severely stressful. Personally whenever someone has a shark (red tail or rainbow) I tell them to try and avoid any other bottom dwellers if at all possible to avoid any disputes and just to play it safe. I'm talking about the 55 not the 20, in the twenty you could definitely do a pair of your choosing and some other fish as well.