No convicts or any other big cichlids. That would be WAY overstocking.
One convict should be in a 40 gallon MINIMUM and 50 gallons for a pair.
You can get 2-3 german rams cichlids. They are only about 2 inches max.
Or you can get:
-one betta
-6 or even 12 cherry shrimp (don't worry, they're tiny)
Or
4 platies
Or
One molly. I'd do no more than one. You can get a molly and some other one inch big fish IF they can withstand aquarium salt. Mollies are brackish water. Meaning they are half saltwater half fresh.
Something different you can get could be a crayfish. They keep to themselves (SOMETIMES). But some should be housed alone.
SOMETHING NOT to get is a goldfish. NEVER EVER get one. For instance, a comet goldfish needs to be in a 55 gallon alone. A fancy goldfsih should be housed in a 30 gallon MINIMUM. Just don't get goldfish. Take my word for it. They're a hassle, they need LARGE tanks, and they need a strong filter.
REMEMBER: CYCLE your tank fish and TEST your water!
As for cycling, I recommend the fishless cycle. Since that is the safest way in my opinion.