I have used them on 19mm wheels with no shims under the knuckle. Perfect height honestly. But, you don't get many runs on them before they wear too small. I use 19mm front wheels on most of my cars besides kyosho stock class where I use 20mm because I use ASC wheels. 19mm should be permitted, as diameter is not officially restricted, but there is no drill for or against the wheel size. I currently have an R8 2015 setup as my kyosho stock car, so don't really have an issue with clearance with k30 low profile front and k20 rad rear.

the compound feels similar to the k30 radial after cut down, which is a little softer than the standard 30.

what I mean by traction changing over long run, is that they tend to gain traction really quick early in the run, and then the balance changes a little mid run to late. Sometimes reaction fades a little, sometimes it rises more than anticipated. With the standard 30, it comes up in the first minute, then kind of stays there. Probably because there is a little less traction overall, and you're always trying to work the car to generate traction. I feel that on high speed layouts, when the 30 works well, I prefer the old cut down with a little more aggressive front end. On a medium or small layout, I almost always prefer the low pro or radial cut down. The difference is VERY minor, and most wouldn't recognize it.