Can small airplanes be towed (or carried) as cargo to be dropped at some other location by air?
If you are talking about externally carrying/deploying other aircraft, this has already been done with the B-52 (albeit for testing purposes) carrying the X-15.

I'm not too sure about having an aircraft drop out of a cargo bay such as the C-130.

On a side note military gliders were used extensively during WW2 and some were capable of carrying light tanks. The gliders were usually towed behind larger bombers to give them the required altitude and speed. They were used because of their wooden construction allowing a stealthy approach due to the reduced radar signature.