BABS - good luck with your tang - As I said, I love the hippos. They can be a little skittish until they get use to all the movement of ppl around the tank - and they stress a little on transportation - but are very beautiful, have great coloring, are not that expensive - and can really stay healthy once you get them acclimated and stable.
Regarding live rock - not much "BAD" can be said about Live Rock! Whether you have a fish only tank OR coral tank - whether you haev crushed coral or a sand bed - whether you have a wet/dry OR just a sump - LIVE ROCK is nothing but excellent for a tank!!
It acts as a great filter that is in your tank at all times (doesn't depend on water being cycled out to reach ur wet/dry); it introduces all kinds of life; it supports algae growth that help w/nitrates AND give your fish something to eat on!! And tangs are veggetarians - love to graze on algae!
I would DEFINATELY start looking at the some live rock - HOWEVER since you already have some fish - make SURE it's "CURED" and add it slowly - 1 or 2 pieces at a time - no more than maybe 15lbs at a time.
ALSO.. the hippo (and other fish) need hiding places and caverns to swim around. Just can't say enough positive about live rock. The ONLY CAUTION - is make sure you don't keep your lights on for too long (over 12hrs) or you can create algae blooms. But I would definately begin to think about adding Live Rock!
BCARP:
REGRADING CLEANERS (shrimp or wrass) they have an amazing relationship with other fish - they eat parasites, bacteria, etc off of other fish. Many fish (especially the hippo tang) will even seek them out and lay themselves out in front of the cleaner to let them eat off of their bodies. (sounds kinky eh? lol)
As far as "CURING" ick - i agree, they probably can NOT do that. I beleive it's more of a preventative that a cure. Fact is, ALL fish have ick - just as all humans have bacteria and viruses in our bodies. It is only when you have an outbreak of their production that overwhelmes your immune system that you 'get sick' -- or for a fish "gets ick". The key is to help maintain good water conditions, and reduce fish stress - AND the cleaners help this with their cleaning rituals.
HOWEVER, if a fish gets ful blown ick - I doubt the cleaner will do much more than have a good food supply, and slow the process down for the fish. It will 'likely' take medicating (and of course fixing whatever the cause was). Most fish that get ick begin scratching their bodies against rocks and surfaces they can find.
In the 3yrs or so I had my hippo in the past he got ick 2x that i can remember. I did nothing, as i just monitored them and gave the shrimp (AND Cleaner WRASS i had) time to do their job - and he eventually got better in a few days. I personally hate to medicate my tank - and didn't have a hospital tank setup - although I would have done so at the risk of losing my fish.
Now, just because MY EXPERIENCE was that the cleaners helped ick on my hippo is NOT a statement to say it's a cure - more than likely, it was just a mild case with a healthy established fish that the cleaners were able to combate it aggressively enough BEFORE the ick outbreak really took over the fish.
All that said tho, I just like the shrimp, and watching the two interact with one another. The cleaner shrimp, LOVES to be on rock "upside down" a lot. so you would definately need some rock, and preferably enough to have a cave so he could walk around upside down
Best of luck to you