Back to the original topic, is the Blue Tang an Atlantic, Powder or Blue Hippo (Regal) Tang? While I would say the the Blue Hippo would certainly outgrow the 80 gallons, I would suggest that it could live with the Yellow Tang without issues until the time for upgrading or removal. And juvenile? This is an arbitrary age, what are the lengths of the fish and what are the dimensions of your aquarium? Maybe there is a need to remove one of these fish, not that anyone here is suggesting a fire, but I don't want the OP to think that there is an eminent danger to his fish until we have asked and answered all of the questions. Depending on the dimensions of the tank and the size of the fish, I may end up suggesting keeping both for the time being. If this is your first salty, you will most likely end up in two categories...
1. Practically giving it away on craigslist just to get it out of your sight.
2. Becoming very interested, involved and craving an upgrade (this is where most of us end up).
As for the LFS, just do your own homework and compare it to the knowledge that they seem to offer. There's so much turnover in pet stores that you'll often find that there's one or two employees that will offer good and honest advice and the other's are either just learning and are just trying... albeit trying on your dollar.
Good luck and welcome to the hobby (addiction).