Any of these fish would be fine in a 65: dwarf angels, hawkfish, wrasse, clownfish, blennies, gobies, basslet, butterflies(very hard to keep), porcupinefish, small puffer, small to med lionfish, cardinalfish, pseudochromis.
Tangs generally need a 4 ft long tank (check out my question on yellow tangs near the bottom of the page)
If you want an inch rule go by 1/2 inch per gallon. I have found this to be faily accurate in community fowlr. Also the better filtration you have the more bioload you can support. If you want a lot of fish it would be a good idea to get a protein skimmer. Also when calculating inches use the fish's full size ex. Flame Angelfish: 4 inches