Will Stain get One for All ability?
One of the requirements for getting One for All is intent/will - both the person passing it on needs to want it to be passed on and the person getting it needs to want to have it.

As any fight between Stain and Midoriya would lack this component (Midoriya wouldn't ever want to pass One for All to Stain - and Stain would have to know about it and want it), there is no chance of Stain getting One for All.

This has been explained in the manga, which is ahead of the anime (and was likely only explained because others have figured the same loophole as you have).