1 - Tangs and clowns are fine, but even the biggest tangs get somewhat large once they're full grown. Plus, they appreciate a lot more swimming room. Finally, they are definitely waste machines. I won't say that a small tang can't be fine in a 50 gallon, but I would still ask you to reconsider your choice.
Damsels and clowns are a recipe for disaster. Damels manage to fit a lot of aggression into their little bodies, especially towards fish with the same body shape (taxonomically speaking, clownfish *are* a type of damsel). You might be able to avoid this, however, if you get blue-green chromis (Chromis veridis), which are another type of damsel. There's never a guarantee, but your chances for peace are much, much higher.
If you must get damsels and clowns, add the damsel to the tank last. If you get chromis, then add the chromis before the clowns.
And, if somebody tells you that clowns need an anemone, just walk away. They live perfectly normal lives without one, and sometimes you can't even get them to host in an anemone.
2 - Well, what's a "real" coral?
Many corals require strong lighting and chaotic water movement (i.e., not just a powerhead pushing water laterally against it), and that's in addition to excellent water quality and the need to dose supplements.
For your first set-up, I advise avoiding corals. Get your hands wet with easier stuff (pun intended).
And, for what it's worth, watch some fish sometime. They don't give any noticeable differentiation between artificial environments, such as tank decorations and equipment, and the "real" stuff, such as rock work and corals. People differentiate between the real and the manmade, but fish don't (they probably can't). Eels hide in narrow PVC pipes, hawkfish perch on powerheads, and everything hides behind cheap plastic decorations.
3 - That depends on what the pump is for and how far the water has to go.
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You're doing the right thing by soliciting advice. My recommendations would be to join some online marine fish forums and grab a couple of books. "The Conscientious Marine Aquarist" by Bob Fenner is an excellent resource to start you out.
But for now, keep things simple. You'll be working your way up to more difficult animals (such as corals) before you know it.