1. This year the color shouldn't matter a whole lot as we arnt sensing anything on the carpet anyway, but the texture is pretty important. At least try and get something close
2.The texture on a shag carpet is far different from the playing field and may effect how your balls/robot perform on your field in comparisson to the actual field
3.They're typically pretty similar
4.If your using them in the fashion that I think you are, its okay, but there are better solutions
using "omni-wheels" for instance is one of them.
both the AndyMark "trick" wheels and plastic omni wheels should work
http://www.andymark.biz/products.htm
5.Very, but do NOT discount torque and traction in the process. Year after year, the teams that win almost always have robust, powerful, fast, and effective drivetrains