Advice- 3.5" Powder blue Tang w/ 1.5" Regal Tang
 

No.

Mixing regal tang with powder blue is ok, but 80 gallon is kind of small for both of them.

Bad idea...

I've done it in my 210g reef, but mixing two ick-magnets is not an ideal situation. (Have 80w UV sterilizer on that tank).

No.
You attempting to justify the purchase of two tangs that require 6 foot tanks by buying tiny ones.
Picture a pie plate in your tank. That will be your regal soon.

If your LFS knows you have a 90 and is still trying to sell them to you, change LFS.

Thanks for asking before you buy.

I'll echo what has been said. I've got a 120g and would never dare put a PBT in there

Best to hold off on the Powder Blue till you upgrade to a much larger tank.

Some tangs are just too easily stressed and become ick magnets, especially when crammed in an undersized tank with another ick-magnet.

PBT have lower survival rates than some other hardier tangs, like YTs, Scopas, and Purple tangs for instance. They are just too easily stressed unless you get the exceptionally hardy one. That whole group of similar looking tangs (Achilles, Powder Blue, Gold Rim, Powder Brown) are notoriously hard to keep healthy and ick-free.

BTW, tiny tangs also have a poor record for survival compared to med. sized ones.

Not what you want to hear, but...

Definitely wait for an upgrade, while the powder blue stays at a "decent" size (ie not as large as most tangs), the regal will grow like crazy and likely get to about 6" in a short amount of time as they eat like pigs (I would guess less than a year). I got mine when it was a few years old and thought I would be safe but he's still growing (although he's slowed down some) and is now about 7".

If you're not thinking about upgrading its likely you'll have to rehome the regal in a year or so as it will be too big for that tank. Then you will be able to think about adding the powder blue tang (and not with any other large tangs).