It should be fine for either one, when you are looking at a fish in your LFS, look to make sure the eyes are clear and the stomach does not look pinched. Make sure it is eating (not just brine shrimp), ask them to feed pellets, mysis, something different than brine. See if they will hold it for you (maybe 7 days) before bringing it home or ask the date they got the fish in, if it has been more than a week and it looks healthy go for it.
I just brought my cowfish home this past Sunday, My LFS brought it in, I worked with this fish for 2 weeks trying to get it to eat, I took in my own meaty foods and bought my Nori and clip from them to feed this Cowfish. I left it there another 2 weeks feeding, so it was there a Month before bringing it home. It is now in quarantine here, of course it ate right away because I was the one feeding it.
If a tang what would be a good one to go with what I already have?
I would avoid the large tangs such as Naso's, the Powder Blues/Browns, Achilles and so on.
Maybe stick with a yellow, Kole, Chevron, Mimic or even orange shoulder.
Kaye