I have a Koran Angel, Yellow Tang, and Kole tang in a 125g tank. Originally, they were all in a 75g but I had to upgrade because these fish are getting quite big.
When I first added the Koran, the Yellow picked on it relentlessly. I had to tear the tank apart to get the Yellow tang out. I banished it to my 20g sump for 1 week, enough time to let the Koran settle in. I then put the yellow tang back into the main tank. All has been peaceful. With the two tangs you have in your 90 gallon, I don't think you will have room for a large angel fish.
[ October 22, 2001: Message edited by: Amphiprion ]