Hey! Yes the sohal is aggressive, and clown tangs can be difficult (lol). Size does matter with tangs so buy accordingly. The clown should be slightly larger than the sohal and introduced simultaneously if possible. If not, put the sohal in last as suggested. Rearranging the rock can help or take out the most aggressive tang and QT for a few hours then reintroduce. No matter what you do - tangs love to duke it out at first to find out who's boss. After that (and some fin sculpting!) they'll pretty much become buddies (within the pecking order that is!)after a coupla few days! I have a sohal, a kole, a blue eyed kole, a chevron, a tommini, a powder blue, a purple, and a Hawaiian sailfin. Planning on a vlamingi, a clown and a desjardini in the near future! They always have some "conflict" with the new kid on the block - but as I said - just match the size of the new guy with the aggressiveness of the tangs in the tank, taking into consideration how aggressive the new guy will be when the smoke clears! Tangs are way cool and the best form of algae control in a high nutrient tank! Just make sure you give em plenty of algae based foods (especially the clown - which is why some find them hard to keep). Mine really tear up 2 little fishies purple sheets, emerald entree, and omega 1 kelp pellets, plus whatever I feed my other inhabitants! Try to stay away from babies though- they seem harder to keep as far as tangs go.
As far as ich - in my experience - hyposalinity qt seems best. The wholesaler I get my livestock from uses hyposalinity for all his fish. I never have a problem this way. A cleaner goby, shrimp, etc., goes a long way too. I'm one of the lucky guys who owns a bali cleaner wrasse that eats other offerings and constantly picks on the fish and my cuticles!!! I have this wrasse for a year and a half which is not the norm, but he always picks at all the tangs - maybe its what keeps him alive!!! Hope this helps!!! Enjoy your tangs and fare them well!!