Sara,
There really is no way to give you definitive answers to your questions. For example, my 75 gal reef houses three fish. That's right - 3 fish. A Passer Angel, a clownfish and a royal gramma. That's it. My 50 gal houses four fish. A trio of saddleback clowns and a green chromis. My 30 gal houses 5 fish. A pygmy angel, a trio of firefish, a six-line wrasse.
Tangs are a touchy subject. Depending on who you talk to. Some insist no tang should be housed in anything less than a 6 ft. tank. Others keep small tangs in 30 gal tanks. IMO, the recommended minimum tank sizes you quoted are just that - minimums. That said, if you got a very small yellow tang or small hippo tang, I'm sure it would do fine in your 75 gal for quite some time. Do NOT purchase medium/large sizes. Since they are different body shapes, you could even house a small yellow tang AND a small hippo tang in the same tank...assuming you had small specimens and were prepared for the future when they outgrow the tank. You couldn't, however, keep both a Naso and a Hippo as they have similar body shapes.
My very first saltwater tank was a 55 gal. I kept a clownfish, Flame Angel, Koran Angel and a small Yellow Tang. Very colorful tank and the fish got along very well. What you need to decide is what you want in the tank first. For example, I can't keep any feather dusters because my Purple Tang used to nibble on their caps and my Passer Angel just thinks they are frilly food. LOL
Saltwater tanks are not freshwater tanks by any means. You simply cannot stock them like you would a freshwater tank. If you are one that likes to look into a tank and see lots of fish swimming around, you will be disappointed. Unless, of course, you plan to only keep very small species of fish and fill the tank that way.
Assuming you get a trio of clownfish, a trio of firefish, 2 or 3 gobies, and a yellow tang/hippo tang - your tank would be fully stocked IMO. You could put more fish in, however, since you want a low maintenance tank without having the expense of $500 skimmers, etc...then you need to stock lightly.
Tom