Hippo, Yellow and Sailfin in a 150?

[Werbo]

Truth is, that you really won't know until you try. Every fish's temperment is different. Yellow and Sailfin are from the same family (same body shape) so you may have a problem, especially if one is larger and the other is submissive. If the Regal was really small you might also have a problem.
If they are all roughly the same size and they are all added at the same time then you should not have a problem. Maybe a couple touch and go days until the "pecking order" is sorted out especially at feeding time around the nori clip. 2-3 nori clip feeding stations would also help. Tangs will always be more aggressive to their own species so a couple mirrors work to distract the fish into thinking that the their reflection is the same species in the tank. Couple ideas for you.
TW