For a mirror floor, just merge it in and then use the scale tool to make it bigger, or do as you said and scale the cars smaller... but scaling the floor larger would be the easier thing to do.
The mirror floor is an object just like anything else... it can be moved (be careful not to move it below ground zero) and it can be scaled as well using the uniform scale tool.
Looks like you are doing good to me...
As for the 2 different chevy's, you can just use the eyedropper to pull the paint scheme off 1 of them into an unused material ball. If you change one, and it makes the other car change as well, then rename one of the balls chevy2 and then click and drag the chevy ball to one of the bodies and drop it, then click and drag the chevy2 ball to the second one and drop it.
You can rename any ball by selecting the material ball and then in the box where the name is right next to the eyedropper, just rename it there.