Early in the round I sent Princess Leia on a mission to an area not occupied by Imperial forces...the Rebels had a few ships in the system. My opponent sent Palpatine to oppose my mission. Then on his turn he used one of the Admiral leaders to activate the system and bring Imperial forces in to initiate combat.

At this point he had two leaders in the system, and I had one. Am I allowed to send another leader there as part of the battle, or am I stuck with just the Princess Leia that was already in system?

no. Rr page 4 "Add Leader: If a player does not have a leader with tactic values in the system, he may take one leader from his leader pool and place it in the system."

Ok, so if you have a leader with tactical value already in system, you can't add anybody then.

correct

And your opponent uses the highest space and highest ground tactics value between his two leaders (potentially one from each). They don't combine.

Ok, so if you have a leader with tactical value already in system, you can't add anybody then.

That's the fact Jack, but you could add a Leader if it was a mission revealed. One of those differences to keep in mind.