IME ich is usually secondary to other diseases, not the other way around. You can run a pretty barren tank for the clowns in the HT. I would include a couple pieces of PVC piping for them to hide in, maybe one T and one ell of the appropriate size for you fish. As for filtration it depends on what you are treating them with. Some medicines will kill off any biological filtration you might have. If you are using meds that kill off your filtration use ammo-lock, am-quel or the like to keep ammonia in check plus do a lot of water changes (10-20% daily) with newly-mixed saltwater. Most parasites will die out if they are denied their hosts (all fish, not just those infected) for a period of time. You'll have to check with the disease forum but I believe the tank needs to be fallow for 2 months for ich.
If you are treating with a "reef-safe" medicine or one which won't kill your filtration you can slightly lessen the need for water changes by adding a sponge filter.
Finally I would give them a nice long fresh water dip on their way in and out of the HT.
