Good going with the Black Cap Gramma you don't see them very often (mine has a personality all his own). I think a tang would be out of the question, "tiny" Pacific Blue tangs tend to not fare very well and seem prone to HLLE. At the LFS I used to work at we had high mortality rates with very small tangs. The mandarin should be fine, I have a pair and they have been switched to brine, if you can't get them switched to prepared foods there are always copepods available for purchase. Your tank ihas a nice balance and I'm sure if you watch the aggression closely you might even be able to get another fish or two. My 110 is ridiculously stuffed, but I like it that way and the extra work is well worth the aesthetic.
Matt's 110 Stocking List:
1- Regal Angel
1- Purple Tang
2- Mandarin
1- Purple Firefish
1- Regular Firefish
1- Dispar Anthia
1- Black Cap Gramma
2- Clownfish
1- Orchid Pseudochromis
1- Cleaner wrasse
7- Green Chromis
1- Canary Fang Blenny