they should get alon well, You could put a blue tang in that tank, but i wouldnt recommend two because they wont get along. try getting a yellow tang and a blue tang. or something along those lines. Right now I have a Yellow and a Blue hippo tang in my 50 gallon tank, both are about 2 inches at the most, they get along quit well. with tangs you really dont want to get two tangs with same body shape, or they feel kinda threatened and start to nip at each other.
This coming from the person who hatched a bamboo shark in there 50. To lead a healthy life a hippo tang should have 5'-6' swimming space. The 75 could be done by an experienced person, but since this was asked in the new hobbyist section, I am going to assume your new. A 125 would be the minimum I would try. If you are planning on upgrading to a larger tank in the future go for it. Once again, you can try it, but I wouldn't risk it right now.