The rules for the Rise of the Empire expansion state that there is a limit of 8 leaders and if you exceed the leader limit, you must decide on which leaders to keep and which leader you must discard.

If the Rebels have 8 leaders and one is captured, and later can recruit a new leader, can they choose to discard the captured leader?

The rule is:

"If a player ever has more than 8 leaders in his leader pool , he must choose 8 to keep and eliminate the excess (remove them from the game)."

While a leader is captured it won't reach the leader pool until it is released. So if when you release him and all leaders go back to the pool you have more than 8, in that moment you have to discard one.