You have the same rule of thumb I use. 17's for 13" rotors and 18's for 18" rotors. Sometimes brake kit sellers offer a template to check wheel fit. I run 17's with 13" rotors and Cobra style brakes. It fits but pretty close... and works quite well on the track.

As far as tires, generally short sidewall helps handling responsiveness. If you are looking for an ultra high performance handling tire, selection in sizes appropriate for or cars will be better with 18's.

I really like the 13" Cobra based system. Cost is reasonable, it works quite well and can be serviced with readily available parts. Have a look at Dr Diff. He offers the Cobra based system as well as 14" systems and higher end calipers.