Getting a good socarrat (or socarradet) is an art in itself. Hardcore paella purists don't mix seafood with land animals, but it's true that some paella styles, considering this is a "poor people" dish created to use easily available (for the zone) components, and leftovers, mix them. A seafood paella can include mussels, clams, shrimp, prawn, cuttlefish or squid, even octopus, lobster, crayfish (or a variant of it which serves just to provide taste but it's not really eatable). For color you use always saffron. Some other fishes, their heads and scrapes (monkfish is a personal favourite for this) are used to make the broth that we'll use to boil the whole thing once the seafood is sauté with the onion, garlic and other vegetables and you add the rice.
The "land" paella uses basically rabbit or hare, lamb, pork or chicken. Then the broth to boil is made from hen and some bone you have around (jamón is particularly tasty).
I couldn't make a really good paella even if my life depended on it, nor my wife, but I know where we can eat a really good one