Having spent my life working and training various hunting dogs I have come to the conclusion that coming up with a number one Squirrel dog is one of the most difficult things to achieve in the hunting dog world. A top Squirrel dog has to use it's eyes, ears and nose. Squirrels are tricky little buggers, they run on the ground, they climb trees and will go up one tree and jump over to another and another (Timbering), it will take some kind of dog to stay with them.
That being said when it comes to hunting dogs you never know until you try. I have seen dogs that one would not look at as a hunting breed yet they were terrific for certain kinds of hunting. One of the best Coon dogs I ever saw was a non-descript Collie type farm dog.
Top squirrel dogs for the most part will come from breeds that are bred especially for Squirrel hunting and tested for many generations. Cur breeds and Fiest dogs are at the top of the list and are what most serious Squirrel hunters train and use.
I have trained some of my Airedales to hunt Squirrels with reasonably good success, but many do not make it because they do not have the strong treeing instinct like the breeds mentioned above or some treeing Hound breeds especially on game that is very small like a Squirrel. I have a pup right now that is going to be tested and by the end of Squirrel season I should know if he has what it takes. The number one thing is getting them in the timber where you know there are Squirrels and observe.
Below is a couple of photos, the blond dog is my buddy Pete Bassani's Kemmer Cur, he came up to my place for a hunt and to pick up an Airedale, she was a real good one. The other is of my TJ he was a pretty fair squirrel dog in his own right but not in the Kemmer's class.
I can say this hunting with a good squirrel dog is about as much fun as it gets.
Also I posted a little video that gives a short synopsis on training, there are many others on youtube if you get serious.
Good Luck
Al