in a 600 I would not worry about it, as long as the tank has good filtration (a good protein skimmer), good water movement, and partial water changes are done regularly.
Convicts are about the same size as yellow tangs, but not as hardy in my book. Purples can get a little larger, and be more aggressive than either species, but if they are all added at once it should work fine.
I would add a bunch of three inch specimens and let them grow up together, no guarantees, but it should work fine.