Either one can be added first. They will not cause any trouble to each other. If push came to shove, I'd add the hippo first, but IMO you could go either way without trouble.
Zebrasoma tangs like yellows are almost exclusively herbivores, which need to be frequently fed vegetative diets, usually seaweed sheets. These can be purchased from the pet store at ridiculous prices or from the sushi counter at the grocery store for cheap. Even though they are big algae eaters, they'll still grab meaty foods when offered, but they will not stay healthy if only offered meaty foods.
Hippo tangs also eat a lot of vegetation, but they also eat a lot of meaty foods. The diet for the yellow tang along with the meaty diet you feed the rest of your fish will do them well.
One word of caution... your profile says you have a 75g tank. That is not the ideal sized tank for Tangs. Many would say it's downright too small. Ideally you want something 6' or longer to house them. If you plan to keep them in that size tank, get very small juvenilles and have a plan to either upgrade tanks or swap them at the LFS for smaller specimens as they grow.