How long have you had the powder blue? IME, they're ich magnets, so make sure you quarantine your new purple, or the stress is likely to cause an outbreak (I asked how long you've had it to know whether it's well-established and likely to be able to handle the stress of another tang).
Your tank is large enough, and it will probably work out, but it will depend upon how established the smaller tang is, and whether your rockwork allows some "out of sight, out of mind" time for any fish suffering from aggression.
Kevin