you should be fine. im warning you now though that the damsel WILL try to protect his teritorry hes deffinatly going to chase the new fish around. a clown would work out real good though alot of people have clowns in a 12 he should be happy. the neon goby should be fine to, its such a small fish its alright in alot of tanks. a blenny, depending on what kind of blenny you want, i would recomend the lawnmower blenny cause it will keep your sandbed pretty clean and with eat most the algae in the tank its temperment is also peacful as well as the other fish i'v listed. if your going to get all three of these i would recomend getting rid of the damsel as nice looking as they are thay can cause major strees on other fish in some cases. even with the 3 fish above your on the verge of overdoing it with your bio-load but if you keep it properly skimmed you should have no problems.