Sorry, i don't know, but i'm sure someone around here will, but in the meantime, it is amazing what one can learn online, simply by typing in, "rat terrier"....1000s of sites will pop up. If you keep hearing the same thing over and over as you peruse the various sites, i'd listen to it.
But, it IS way more fun and insightful to hear directly from those who live with the breed, of course, every dog is unique.
But, that said, i actually only meant to post, that i want to commend you for looking for your dog via SHELTERS or RESCUES!! Good for you!!
Another huge advantage is getting a "rescue" dog, is the foster fam can tell you all about that individual dog, which is BIG PLUS since you have a specific type of dog you are searching for!! I got mine from a pound, (shelter) and couldn't love him one bit more than i do.
I find it kinda charming that you are seeking such an energetic dog, how great, some people look for mellower dogs, so it's interesting to me to hear from one who is going for those real high energy dogs!
Petfinders.com is great website for searching for your next fur kid when you are ready. This website combines both rescue and shelter dogs. Some rescues will help you "temperment test" a shelter dog, some shelters may be able to do this, as well. I'm not entirely certain if temp testing helps determine a dog's energy level, though...not sure of that.
GOOD LUCK! Stand by, someone who knows about Rat Terriers will probably be by!! lol!
but i just wanted to say, how wonderful you are not gonna support a puppy mill, and are going to save a homeless dog!! YAY!!