I had a kole tang in my tank which I added in early because it is believed they are one of the less agressive tangs. Anyways, I added a flame angel, and he bullied it to the death. I added a yellow tang, and he kept chasing him all the time. The yellow tang lasted a month before it finally died. I finally got rid of the kole tang because I didn't want to keep an ugly fish that terrorized everyone. If you feel that your kole is mellow, then you might be able to get away with adding a yellow tang. As for compatibility, I think a purple tang (or gem tang, and few rare others) are the only ones who have a similar shape to the yellow tang and should be avoided.