Best Cat skid to use, is the 1M, 03 and newer models. 03 use's different rails and 6 1/4" rear idler wheels. 04 and up use 8" rear wheels and the rails have a single point bend at the curve. 04 rails are suppose to be lighter, however the 03 rails are stronger.
The 1M skids are lighter than the 02 and earlier skids.
Not sure about the RX/RS chasis, but a Viper tunnel is 3/16" narrower than the Cat tunnel. So I had to remove 3/32" off the end's of the front arm and inner sleeve's. It's easy to do with a stationary belt sander though.