Can the overlord immediately place and activate a defeated monster?
I would vote for: Yes!

I think the real question is, can the Overloard place it directly on his turn, or does he have to wait until the next round? I think there is nothing in the rules that presses him to wait. I think the rules for reinforements are clear about when they occur, they are usually given in the scenario description.

If there is a monster on the board, the Overlord can activate it. There is no summoning sickness.