7 fish? ok, ill seeI would say any wrasse is too big except for possums, but those are expert level fish most of the time. I would say the goby, couple Firefish, a clown or pair and you’re pretty stocked. The clowns will be highly territorial in such a small environment.
HOBs work as long as you have an extra flow pump/Wavemaker in there. A small canister filter would also work