Frozen shrimp are OK. In fact, frozen shrimp are usually better quality than fresh shrimp unless you have a local fresh shrimp fishery or local indoor shrimp farm nearby.
For a really great shrimp cocktail, you want to go with large shrimp. I use a 21-25 count, which means there are 21-25 shrimp per pound. Buy them deveined but uncooked. Easy-peel or already peeled is fine.
If you think the shrimp might be sitting out at room temperature for a while, it is a good idea to present the shrimp on a bed of ice cubes either on a platter or a shallow serving bowl. Make sure to have a bowl of lemon wedges on the side or you can place it on the platter with the shrimp.
The answer largely depends on two factors: how large each piece of shrimp is and how you are using the seafood in your meal. A good rule of thumb is to plan for a quarter-pound per person for an appetizer, or a half-pound for a main course.
Steaming is the best method to cook shrimp because it doesn't overcook the shrimp as quickly as boiling water would.