Thats the Russian experiement isn't it?
Wolves and dogs can be interbread. I don't remember about this fox thing.
I suspect that isolated populations may take a considerable period of time before breeding becomes completely impossible.
The subject of mules relates here.
It's really up to the evolution critics to propose some mechanism that prevents two isolate populations from becoming reproductively isolated, as with horses and donkeys.
It is also part of the unfinished business of evolution to explain why some lines, like dogs, have more variability than others. It's an interesting problem.