I always thought fiberglass was fire resistant. To a certain point that is. I'm a bit leary about putting the plastic up there, since you get get moisture behind it and that could lead to a serious mold problem which is something very nasty to get rid of. And as for the floor, it looks to me like what you see there is the upper side of your drywall ceiling. So, I wouldn't put much weight on that or else you are gonna have a big surprise in the room below. You could very easily build a floor from rafter to rafter using some plywood, and that would solve your problem. But I definitly wouldn't set anything on the drywall. But otherwise, this looks like a really good place to grow