Here's the thing about this question; you're going to get a lot of varied opinions from people who have experience with a lot of different scenarios. Most research and personal accounts note that tangs require a fair amount of swimming space when they're adult age. So, if your question had been "can I keep an adult hippo tang in a 55g?" you'd probably get a lot of people screaming no.
Here's my take on this: a juvenile will do fine in a 55 gallon until it reaches a few inches or reaches about 2 or 3 (max) years old. After that it will most certainly need more swimming room. Chances are that you'll grow attached to the fish and won't want to give it away, so here's my suggestion: if you plan on upgrading to a tank that's over 100 gallons (125 or more is what most people will say) sometime within 2 years of getting one, then go for it. That way you know your long term solution won't involve giving it away when it gets too big for the 55.
Now, I don't personally have experience with tangs, though I've done a fair share of reading up on just this very scenario and since I'm going to start on a 180 next year I might just put a juvenile in my 55 to get it growing until the 180 is ready for it. Numerous people have noted that juvenile tangs may be better off in smaller tanks. Anyway, if you get the tang and in 2 years don't upgrade, just be prepared to give it away. Hope that helps.