Yes they should be fine. Most other Tangs don't bother Hippo Tangs, difference shape and color, and even though some Hippo Tangs can be aggressive, most just go about their business and don't bother other fish, unless it's another Hippo Tang.
No, but you need to make sure the Marine Betta has caves and places in the rock where they can hide, as they're pretty shy. You just need to worry that they may pick at its fins, usually the tail. Also they don't do well with very bright lights, so shelter is important. Marine Betta may also eat shrimp, so be careful.
Yes, they're very hardy. They're groupers, Round Heads, and will live a very long time, easily over 20 years. They usually don't come down with diseases and are very hardy, eating just about any type of food you may put in the tank, though they're somewhat shy and you need to place the food near them.
They are very hardy. But they don not seem to like strong currents. It also might take awhile to get them to accept dead food at first. You might want to be more careful of the tangs. Blue hippos are known to be ich magnets.
Marine Bettas are good strong fish, the only problem is that as already stated they are very shy. You will seldom see it. So if you can live with that they are beautiful fish. Mine is easily 9 years old. Poor thing got attacked by my puffer 3 or so years ago, and I was sure it was a goner. Happily it not only survived the attack it has no scars.
As to the tang choices you should be fine as already stated.