Ok Piggy,
I would be remiss not to give my experience with Tangs. The family zebramosa tends to be the most aggressive of the tangs. Blue tangs are the most passive. If you want to get more than one tang, add the smallest, least aggressive first and increase in size and aggressiveness from there. Tangs get super territorial and in a 55 you will not be able to have more than one. Often times I see baby Regal Blue Tangs for sale that would be OK in a 55 for a year or so (I assume you meant Regal Blue Tangs as opposed to Atlantic Blue Tangs, I have had both). Even if you upgrade to a 90, I would not try to house more than one tang. They need length of space to create territory and four feet long is just not enough for multiple tangs.
That being said, you can choose to add about half the different species of tangs in a 90 gallon, as long as they do not reach sizes bigger than 10 inches.
Blue tangs and yellow tangs tend to be more passive. Purple, sohal and powder blues are nasty fellas. Orange shoulder tangs would be OK in a 90 and are not often seen in tanks.
You mentioned wanting to upgrade to a 90 and that space is limited. There is another tank option with a 120. They are 4x2x2 and take up virtually the same space. I would still not try to add two tangs to that size tank, but would give you the option to have more fish and a tang.
Hope something I said is useful.
Yote, 200 is a great size tank, but I think a Regal would be Ok in a smaller tank. My guy is in my 90 right now and has shown no signs of stress. I don't even have a UV sterilizer on the tank and he has no Ich. I think he will be OK.
-Dr Marco :sfish::sfish: