IMO a 58 is fine for a small tang as long as there arnt too many other fish as well. When it gets big it will need a larger tank though. With the only other fish being the maroon clown it should be ok. Maroon clowns can get very territorial as they grow though.
I tried Kick Ich twice, the first time the ick was bad and the Kick Ich worked very well. The second time I tried it it did not work at all, I saw no difference after using. So IME it was 50/50. Any other time since Ive seen a fish with Ich I feed heavily usually twice daily with food soaked in Garlic, like garlic extreme from Kent Marine, and also soaked with selcon. I reduce any other stressors I can see and it almost always clears up. Last time I got a new fish, a foxface, it freaked everytime it saw me. Developed ick right after arrival, I realized all the action in the living room stressed him so I covored the tank in trash bags for a week. Now he is nice and healthy,and the trash bags are gone.
Maybe Im crazy but IMO if you reduce all the stressors you can it really helps increase the chance for recovery. HTH