Compositing video layers in 3D space Here's what I need to do.

I have a footage of a street. I want to add several people walking. Each person was shot using chroma.

I understand how to remove the green using nodes. I understand how to composite each element using nodes. What I don't understand is how to I manipulate them in 3D space in order to move them where I want each element to be or animate them.

I could use the video as texture on a plane and move the plane around. But I want to know if I can process that video as texture using masks and keying. Or should I key every person, render them, then add them as texture to the planes and work like that? 

Thank you
You could perform the keying in the compositor scene. Create a 3D scene to reproduce the depth of the street shot. Then place UV unwrapped cards in that scene, each card could have a layer pass.

Back in the compositor apply the alpha keyed image to the 3D scene layer using UV distort. It should scale and map the people to the card.

Here is an example I made at the forum http://blenderartists.org/forum/showthread.php?332091-Card-Compositing-to-simulate-depth