An anemones require very high lighting (ie. expensive). It will also need a very stable, well established tank. Most recommend waiting at least a year to add an anemone. Keep in mind anemones move around your tank to find their happy place. You won't want other corals in the way that it can sting. Our clowns have hosted zoas, hammers, and frogspawn. All of these should do well in a fairly new tank and with t5 lighting. You could add them a month or two after your cycle is finished and they would do fine.
Hippo tangs are beautiful. Unfortunately a 72g is really too small for them. They recommend at least a 150g for a large hippo. It will take years to get to that size but eventually will need a very big tank. You could get a small hippo but you will need to keep in mind you will eventually have to upgrade your tank or get rid of the tang. Also, small hippo have a very low survival rate in captivity. You would have better luck buying one a little bit bigger, but you couldn't keep it as long. Personally, I get too attached to my fish to buy them knowing I will have to get rid of them. If you have your heart set on a hippo maybe you could start with a larger tank. I would not go any smaller than a 90 and that is not really ideal for a hippo. You will be amazed how fast your tank will fill up!!
It is hard to say how many fish you can have in a 72g. There isn't a formula you can use. Many factors to consider: shape of tank, how much live rock is in it, temperament of fish you have. I would recommend looking through this site and LFS and seeing what you like. Make a list and run it by people here. Some prefer larger fish and that would mean you could have less. Some people like a tank full of small, colorful fish. Others like to have a "show piece" fish and then several smaller fish. Personally, I like one or two show fish and then lots of small, colorfull fish that move in and out of the rocks.
Substrate. It is all a matter of personal taste. Bare bottom, sand, agronite, or crush coral (not usually recommended because it tends to trap food/waste particles). We have our QT bare bottom (always recommend that), two tanks with sand, and two with agronite. If you have your heart set on a fish that is a sand sifter, than obviously you will want sand. I will mention that we did a tank once in black sand. We liked the idea of it because our tank was black. Our trusty LFS guy told us several times he didn't think we would like it and boy was he right! It looked great until we started adding corals. It absorbed the light rather than reflecting it and it washed out the colors of the corals. We were much happier when we changed it to agronite.
The best advice I can give you is to go slow and research first, which it appears you are doing. I would recommend setting up a QT tank before doing anything else. You can get a cheapy tank, light, and hang on the back filter and get it started cycling while you are setting up your main tank. You will NEVER regret that you did that. If you have a friend or even a LFS that would give you a handful or substrate or some floss out of their filter it will help it cycle a little faster. There is nothing more heartbreaking then seeing a tank full of fish that you have become very attached to die and there isn't anything you can do. If you QT everything first and then keep the tank for an emergency hospital tank you will always have a back up plan. If you use the QT religiously, you may never need a hospital tank. Trust me, I have been there done that, it is not worth rushing things. In the long run it will save you time, money, and a lot of heartbreak. There are thousands of references on this site about how to set up a QT. It is incredibly hard to be patient and QT but in the end it will be one of the best things you can ever do. You will find this hobby requires alot of patience!
