Hello!
It seems the number of fish would be alright if you have a cycled and stable tank. Are you familiar with the nitrification cycle (and never throw out your filter pad or wash it in straight untreated tap water).
A nice top fish would be a few 3-5 male fancy Guppies - only males so no babies!
Or one of my fish favorites which loves the top few inches would be the Neon Green Kubotai / Microdevario Kubotai it does however have the similar body shape to the Cardinal Tetra and again the Emperor Tetra, in case you would rather have a different shaped fish for additional interest.
Rachel O'Leary (youtube aka msjinkzd) has video about them and their care.
If you have a cycled tank, I would also recommend another 3-4 Cories if they are the medium sized ones, like Bronze, Green, Albino - Corydoras aeneus.
After adding fish, (quarantine / QT is recommended before adding to you main tank) just be sure to watch the water parameters as additional fish mean a larger bioload.
Feeding half as much but 2 times per day, helps spread the food and waste load more evenly in the day and make it easier for the filter to process.
You might consider also adding a 2nd weekly water change to keep the water in good condition initially to the 20G high for a few weeks just to make sure the BB in the filter is doing a proper job of keeping the water safe!
