If I were to have a locomotive with a #5 coupler and a car with a different coupler (not horn hook) could they hook up without problems?
Generally speaking and there are exceptions, the KD #5 should mate with the other five major brands.
Operating wise, better performance on 22" radius than 18" radius.
Not all locos use a #5. Go to www.kadee.com/conv/list.htm for verification.
You may find some of the exceptions when it comes to uncoupling. Some may not work well with the KD #% on magnetic uncoupling ramps. Others (or possibly the same ones, frankly it’s been a while since I was in HO [:D]) can be a bear to uncouple manually as well. There’s a “universal” coupler that I seem to remember did quite a good job at matching just about everything else, but had to be sort of twisted to get it uncoupled. (Then maybe I had installed it not quite true? I don’t want to malign an innocent product through my own misalignments! )
I find the Bachmann-type couplers with integral plastic centring springs don’t tend to work particularly well in delayed uncoupling - they sometimes don’t move across to the delayed position properly. All my stock fitted with Kadees has no trouble with delayed uncoupling (mostly #5s but a few #31/#34/#37 offset couplers on certain locos and stock). These all work perfectly together and with the “E-Z-Mate” couplers. I use these on some stock where I can’t fit Kadees without major surgery, and on most Athearn locos as it’s easier than drilling and tapping mounting holes!