I agree 100% with Curiosity. There are some problems with your current fish and until those are resolved I wouldn't feel right recommending you some more fish.
I know it might seem silly at the moment, while the fish are small, young and passive but these fish (apart from the peppered cory, if that is what it is) will get really big and will need a much bigger tank. The two sharks will also end up fighting as they grow older and should not be kept together unless in a truly huge tank (I'd say 70-100 gallons, maybe more).
At most, I'd suggest you get some more of the peppered corys as they should be kept in shoals. Getting 5 more would mean they all play together and swim around which would be great for them and make your tank seem less empty.
If you can upgrade to a 30 gallon tank that is at least 36" long, then you could keep the rainbow shark and add some shoals of tetras/rasboras and some other fish. If you wanted to keep the red tailed shark you'd need to rehome the rainbow shark and get a 40 gallon tank (or kept it with very, very few fish in a 30 gallon) and if you want to keep the clown loach you'll need to be thinking about a 5-6 foot tank in the next few years.
If you just want to keep the little tank you have now, you'll need to find a new home for both the sharks and the clown loach. Once you've done that, a good community stock would be something like 6 corys, 6-10 tetras/rasboras and 2-3 honey gouramis.
Dos your peppered catfish look like this?
Sorry to pounce all this bad news on you as you've just joined. We just want to make sure you get the best advice about your fish and that all your fish can have the tanks they need to thrive. Welcome to TFF!