Tankmate for Henry?

You need at least a 5 gallon to have a tank mate… Also cory cats like to be in groups so a 10gal would be best. You'd need to buy sinking pellets for the cories. I'm not sure if cories can handle salt or not, i suppose that it'd be fine though.

No other fish are suitable to be housed with Henry in a 5 gallon fishtank, other than shrimp. So if you really want something, get shrimp.

The reason why you can't have any more fish is because of the bioload, or ammonia being produced. Most fish need to be in proper schools, and a 5 gallon is not suitabke for schooling fish in the first place, let alone schooling and a Betta. Cories are schooling so you need a minimum of 4. Shrimp have a relatively small bioload. You could get up to 40 at the extreme, though I would recommend around 5-20. Just remember that you will have to buy more food for what ever you put in.

I totally agree with Kfry.

Neither cories nor shrimp are tolerant of salt, as cories have no scales. It is also not a good idea to use salt long-term with bettas, as they build up an immunity to it for treatments, and it can cause them internal damage.

You should always use dechlorinator if you have fish or shrimp in your tank.