I hate to say it, but I think this is a lot more complicated (and difficult) than you might initially imagine. First, you'd actually be dealing with not one but two green screens, as people would presumably be moving both in front of and behind your actor. Either that, or you'll have to do a RIDICULOUS amount of rotoscoping.
And who are the people in fast-motion? Do you have a budget to hire the massive amount of extras needed for this? If not, are you thinking you'll shoot plates in public places? How are you going to control that environment? How are you going to keep the general public from not walking where your actor is to be placed? How are you going to keep the fuzz off your back?
How's this for an alternative? Instead of having all the extras walk in fast-motion, why don't you have them in normal speed, while your main actor is in slo-motion? All of the complicating factors I listed above would be significantly reduced by simply reducing the amount of time you need to record (and therefore the number of extras you need).