Pick two small fish, adult size 3" and under, that aren't especially active. If the tank does well with 2, you could go for 3. A pair of clownfish is always nice and maybe 1 fire fish? or if you don’t want clowns a pistol shrimp/goby pair and then 2 fire fish. 2 cardinals and a firefish could also work. I’d say your tank is a little too small for a mandarin (imo)