If you are competing with meat birds you will not be able to pick the breed. Your chapter will place an order for the chicks and you will pick them up. This way, all the chickens are the same age, theoretically should be fairly uniform, and to eliminate any other problems.
Unless you are in an area with a fairly large 4-H program you may not be able to compete with other breeds. Our show was really small and they had meat classes for the cattle, sheep, goats, rabbit, swine, and chickens. And they only had a breeding class for rabbits and cattle.
Now when we went to San Antonio the shows were huge. The had Dairy classes for cattle and goats. You could show cow/calf pairs, turkeys, ostrich, quail, I mean everything. And to win you had to have the best. At our show, if you entered a breeder rabbit, you just had to make sure that you were the only person to enter that particular breed in the show and you were guaranteed a blue ribbon because you were the only person with tat breed.
So I would advise looking into the show itself and make sure that you can show something other than meat chickens. You should be able to call your County Extension Office and they can find out for you, or point you in the right direction.