Advice? is my reef overstocked

I think that looks good but my own preference is no damsels. I think he will find your chromis and cardinals easy to bully. Just something to consider. Have you considered some type of blenny? I think a midas blenny would be good. They wont compete for grazing rights with your tangs like a starry or lawnmower one will. Or there are the striped fang blennies too if you want something that swims around more freely rather than perching.

 

If you add a Kole tang make sure if you have the opportunity to observe it, it is fat, healthy, and an aggressive eater. Then get several mirrors and position them in front and sides of the tanks. Most of the time this has worked for me adding new fish. The only time it failed is my Purple tang would not stop picking on an Aussie Tusk. I grounded the Purple Tang in a Qt tank for a week, put him back in everything was fine.