There are some maps in campaign in which you reveal an ally in the middle of a turn. My question is: can that ally be activated in the same turn that is revealed?

It depends on the mission rule. I guess it tells you to deploy a group, unless stated differently, that group is deployed in "ready" position.

Cards enter the game ready unless explicitly specified otherwise. So, in the overwhelming majority of cases an ally deployed by a mission rule during the activation phase is a ready group you can activate (and must activate if it's the only ready rebel group).

(Or the the following round if deployed by an end of round event.)

Ok, thank you guys. We played the Gideon mission "Friends of Old" and it seemed too rebel biased to give the trooper one movement with Gideon and then activate the trooper. I was the Imperial and I couldn't kill the troopers because they we're fleeing at full speed.

Note that all Rebel Troopers belong to the same group (deployment card). When you activate a group, you activate all figures in the group in the order of your choice.

Edit: Friends of Old can be over quickly, or be a closer call, depending on luck and the threat level you play it in.