The right reply according to the Tang Police would be that a 90g is too small for any tang, however as I have a 72g with a Sailfin I wont throw stones..
The general feeling is that a tank must be a minimum of 6' long for any tang to do ok. Huge tanks 400g+ are the best. If you are really gung-ho about combining tangs, you have to make sure that you have lots of rock as they will probably squabble like heck for the first bit, the loser must have a place to hide....