Most servers are designed to be right on top of each other. If they don’t have vent holes on the top they can be stacked without any space between them. The cool air comes in the front, so short servers on top shouldn’t matter as the hot air is forced out the back.
You should try to put your heavier servers towards the bottom of the rack though, just to keep the CG as low as possible. And dont forget those metal feet for the racks. So that the rack doesnt fall over you when you pull out a server from the top third of the rack.