You should be fine with 2 hippo tangs in a 150. Hippo tangs are not nearly as aggressive to new tank mates as are tangs such as yellow or powder blue. I have a 270g tank with four hippo tangs of various sizes all added at different times. I also have two yellow tangs, 1 purple tang, 1 sailfin tang, and 1 powder brown tang. Just make sure they are a little over fed the first week and they'll be fine.