Your tanks size is fine.
Its true, tank sizes are a very delicate subject and its wise and very responsible to ask before purchase.
I purposefully bought anl 100 UK gallon tank (120 US gallons) just so i could have a tang.
Please excuse the HUGE blue ring.. it was a mistake from my lfs as they ordered the wrong size and didnt have anywhere to house it as their tanks wernt large enough. I took it to save it and find a new home but alas it died.. too old and wouldnt adjust to captive foods.)
Ok back on subject... this was a large tank and as most people say, its more than enough for a tang in its requirements.
But please remember that most tangs will grow larger than that blue ring so now you can imagine just how big they would look in most peoples smaller systems
I purchased an Emepror tang and my dream of keeping tangs was a reality.
however....
Although the tank was adequate for this fish, it didnt act in a manner that i had come to expect for most of the tangs i had witnessed. It was very jumpy, highly aggressive to anything that resembled its size or colour (tangs are known for their aggression), it harrasssed my asfur relentlessly
It gained the name of "Ming the merciless" (Emperor ming.. emperor tang get it???
) and to this day its the only fish that has ever had a name in my tank.
Now....
I soon upgrade to a bigger tank.. my 220 UK gallon (264 US) and put it in this tank Its behaviour changed almost overnight. It calmed down, tollerated other fish and was far less jumpy. it spent all its time just cruising around the rocks picking on their surface for food rather than darting from rock crevace to rock crevace in the old tank. I added more tangs and although there were some highly aggressive moments when they were introduced, within a day or so they alll shoaled together and i never had fighting. I cant imagine what would have happend if i tried a 2nd tang in my old tank.
Ok so in my experience and opinion, their behaviour definately changes with the surrounding territory they perceive.
I have been behind the scenes at wholesellers and i have seen the holding tanks that many use tangs for. i must say its no suprise to me when people say they got their tang home and they brokeout in ich etc. the stress levels are enormous. I saw a tank that was 2ft x 18`x 18` In this tank was a single clown tang and about 15 small yellow tangs. the clown was far larger than the others and had gone completly insane in its conditions and was just contually chasing its tail. the other fish were all cowering in the corner in fear of their lives should they swim close and the clown lash out.
So the very moment you buy a tang remember that you are already taking a fish that has not had the best treatment (i must say that TMC which is our largest supplier in the UK does give them far better room). and is fighting for its life. Im not suprised at all when these fish are put in tanks and they show extreme territorial rights over a patch.. they have just come from an ocean and then probably dumped into a tiny pudle with 2 dozen other tangs before being released into your home tank.. of course they will now fight for every scrap they can get.
ok im rattling on here but what im trying to say is that yes, you can have tangs in smaller tanks but expect extreme behaviour changes. if you want them to act in a more natural manner then get what truely experienced reef keepers tell you rather than reading commercial sites that want to sell you their fish
Bristletooth are smaller and not as free swimming as other tangs and thus i would say its fine for a tank of your size.
Ok its rather a long post to say all that but its needed so that people can see what happens behind the scenes and the differing behaviour in the same fish with different volumes of water.
I would personally get the Kole yellow eye Tang. they get 7" most places say 70 gallons will be fine for a full grown fish. For what they eat bristeltooths will eat most frozen foods like mysis but feed seaweed and lots of marine algae. it will increase their immune systems which is good cause tangs like to get ich 
