What can you keep in a FO tank?

Hi m8, firstly i can tell you what a tank with fish and live rock is normally called. FOWLR (fish only with live rock)

As far as the rest of your questions, i will leave them for some one that knows more that me ( 99.99% of people here)

FOWLR are good setups.

You do not need to buy all 75 pounds at once--you don't even have to start with all LR. You can easily get 10 pounds of LR, and all the rest tufa--it will become live, both bacterially and in algaes and small corals. It will be slow, but happen. If you get a mix closer to 50/50, it willl happen a bit quicker.

For more than just coralline algae (the purple/green/red hard stuff), you'll need to consider the lighting. Normal Output fluorescents are not adequate for most photosynthetic critters (ie, anenomes and corals)--you need to have HO, VHO, power compacts or metal hallides. But--you can get the tank going, and upgrade onse you decide what corals you want--the rock, crbas, shrimp, worms, snails, and fish will be fine in NO lighting.

The only thing I'll throw in is that you shouldn't waste your money on local fish shop sand. If you're east of the Mississippi check Home Depot for "Tropical Play Sand." Tropical Play Sand is one crystal form of Calcium Carbonate, it will balance your pH perfectly at 8.2, and will provide a great bed for bacteria growth. Instead of spending $20 for a 15 pound bag at the LFS you can get it at Home Depot for $4 for a 50 pound bag, and it's the exact same thing as sold in the fish shops. I have about 70 pounds (4-5 inches deep) in my 30 gallon and it's doing great.