Boy, lookin at this thead you'd think just about any dog with a little bit of chase in it is a potential world champion squirrel dog. It may be possible to get a lot of off-bred dogs to hunt squirrel but doing it well is completely different.
If you're looking for a squirrel dog then get a squirrel dog. Specifically a feist or cur from proven stock. Most dog breeds including the terrier breeds mentioned here are so far removed from hunting stock as to make them a crap shoot at best. They may make fine hunting buddies and may even contribute to the hunt but that's a dang long way away from a good tree dog.
Been around hounds, curs and terriers my whole adult life and I'll tell you this, tree dogs are born not made.
I had a Golden Retriever when I was younger. My hunting buddy and when I headed out I didn't care if we shot birds, rabbits, squirrel or the occasional daytime coon. He'd use his nose on any of them and helped bag some "off" game that I might of walked right on by. But a good squirrel dog would of smoked his butt in the woods.