I'd not do a mandarin as your first fish. Something really easy like a clown goby, other small acropora dwelling goby, or a singular percula clown. I'd also skip the cardinal or firefish (they jump and every time they do it's flushing 10 bucks down the toilet).
The suggestions given to you were kind of bad:
Damsels are highly aggressive and get very large (~4") too big for your tank.
Cardinals are about the same too. Both are very active swimmers. Your only options are the smallest anemone fish and gobies.