Yes, it will be fine. I use a 20L for my QT tank.
I currently have 4 fish in mine. a flasher wrasse, (2) anthias, and a damsel.
have put many fish through 30 days of copper in it.
Just get yourself a Seachem ammonia badge which can monitor ammonia, even when using copper based meds. As long as you keep uneaten food and waste cleaned up, along with small water changes, you will be fine.