I'm going to make another discussion regarding a different topic that I think would work better
Thank you for writing here before I was going to move topics (if you did)
When you say you're making a template, are you saying that you plan on using the source video with the green screen for multiple projects, and replacing the green with something different each time?
If the green screen area is the proper proportion for the video with which you want to replace it, I suggest skipping chroma keying (which you probably thought you'd have to use) and instead add your replacement video as another layer, then add the Quad Warp effect to it. Drag the corners so that it covers your green screen space, and you're done.
To make repeating this a little easier in the future, turn your replacement video layer into a composite shot, leaving effects (the Quad Warp) in the original comp (which I'll call "Main" for now). Name this new comp "Source." When you need to swap videos, just drop the new video into the Source comp. Back in the Main comp, this embedded comp will still remain where you placed it with Quad Warp.
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