Is there a legal requirement for planes to start from the ground?
At least in the US, this is perfectly legal. There have been quite a few such aircraft in history.
However, an aircraft docking and undocking from another is so rare that there's no standard way of communicating it with ATC. So you'd have to explain what you're doing in plain language. (And then you'd have to explain it again when the confused controller replies, &quot;What?&quot;) If I were flying such a mission, I'd contact the local FSDO ahead of time to explain it, so that they can forward the information on to the local ATC facility to cut down on confusion.