It's not really a 6 of 1 vs half-dozen.
- Tangs and Wrasse may die from being caught, handling. But A mandarin or CBB will most likely die from Starvation. It's easy to feed a wrasse but can be impossible to feed a CBB if it doesn't want to take prepared food.
- Butterflies also need ALOT of swimming room. My CBB is more active than my Tangs and will spend hours racing around the entire tank.
- Angels will demolish your ornamental algae, let alone worrying about a tang. The shrimps will also eat some as well.
- All fish can develop Ich. You should be QT'ing your fish properly. Once Ich is in your system then any fish under stress is likely to get it. My CBB has had it, my BT has had it.
- I wouldn't class a Tang as sensitive, Butterfly much much more so.
- Butterflies can also be aggressive to new additions, much the same as a Tang. However, a Tang is better suited to stick up for itself.
- Targeted Vegetarian Food? Thats a terrible reason, you will find you have to get live food to target feed your Butterfly if it decides it's not prepared to take prepared food, or just stops eating it one day out of the blue (as they VERY commonly do).
The white-plague? Errr you have been seriously mis-informed during some of your research.
White-Spot has a life-cycle of approx 6 weeks. Of which only about 2 weeks need a host fish, and any fish will do. Once you have it introduced into your tank then any fish is capable of carrying it. The best prevention is to not have your fish stressed out.
Also, do a search for "Lymphocytis a virus". This is an almost exclusive Butterfly (most specifically CBB) and some Angel virus.
There is no known cure for Lymphocytis. Cutting or removing the cysts is a risky procedure leaving an open wound exposed to a wide host of bacterias or worst still leaving behind some of the cyst to allow them to grow again.
I am sorry, but you really do need to spend a fair bit more amount of time researching the proposed additions to your tank.
While I think a CBB would be a lovely addition to your tank, none of the reasons for it over a tang are valid. CBBs have a VERY dismal captivity survival rate because people treat them like a normal fish, throw it in, add some flakes and it'll be right.. just like a clownfish. Nothing could be further from the truth.
Each night, I have to ensure that my CBB is able to get some food, and is eating still. This requires feeding my tank with a frozen enriched food every day (3 cubes ensures he gets enough with the other fish) after I have fed a spirulina flake food for the other fish. This doesn't help with the algae because of the phosphates and nitrates added. As for feeding my tangs, they can graze at algae off the rocks for days (if not weeks) with no problems. I also spend time each day watching my CBB graze at live zooplankton from the rockwork (200lbs) and ensure that he isn't going to decimate the overall population.