Baykal,
If your supplier is willing to exchange the Kadee no.17s that you have ordered for no.20s, perhaps you should ask him to now, because I don't think you will ever need any quite so short. On the other hand the no.20s will be necessary on your Bachman Mk1s and very useful on other stock to avoid buffer locking. Let me explain. If you imagine the Kadee coupling as a cup on its side, then the bottom of the cup needs to be on the same horizontal line as the face of the buffers. American stock does not have buffers so Kadee do not tell you this.
Most people fit them on the ends of rakes of coaches or wagons, and on locos, because we want the automatic uncoupling using the flat magnet, type no.321, that can be fitted between the rails for delayed action uncoupling. How this works is explained on Kadees web site at:
http://www.kadee.com/animation/c1.htm
I also have one of their electro magnetic underbaseboard uncouplers on my main line platform so that I can change locos without moving the train.
There is also a large flat magnet type that fits under the sleepers out of sight, but I don't recomend this one as the magnet is too powerful and it "grabs" the steel axles of wagons as they are pushed or pulled over it. This would be no problem with plastic axles, but who uses those these days?
Kadees can be tricky, bit well worth the trouble for hands free operation. They are vastly superior to the ordinary British Tension Lock device that was introduced over 50 years ago.
Colombo