[nereefpat]

A 20L or 29 is great size for a qt, as long as you aren't buying huge fish. How big is the display, or what fish will you be quarantining, by the way?

If you want to use a filter in the qt, then the canister will be fine. As you said, carbon would need to be removed if you are medicating. I wouldn't bother with a powerhead (unless trying to qt an Achilles/sohal tang etc). Just point the return of the canister at the surface. I just use an air stone with no filter.

A bacterial starter like dr tim's would be fine. Another way to manage ammonia is just monitor it and do water changes and add Prime as needed.

Another thing...Don't rush. Take your time. Don't add everything at once. Nothing good happens fast here.