N., I've got a rat terrier that we adopted as a small puppy-I was looking for a fox terrier but took the rat terrier instead.
These little guys are stubborn and full of energy. You need to make sure you and the family are ready to devote a LOT of time to making sure that this pet gets plenty of excercise, attention, and training. They are great dogs, very loyal, but you will need a lot of patience and dedication to raise them.
The biggest issue we've had with ours was peeing in the house and agression, that really stemmed from not excersizing him enough and not giving him a "job". We had a terrific trainer come in to help us and he's been doing great, but the first couple of years we had him he just about drove us nuts. Also, he's about 4 years old now so the puppy is starting to leave him, he's a lot calmer than he was say a year ago.
I'll also add from personal experience that the way an animal acts at the shelter is not necessarily what you'll have on your hands once you get home. They are under a lot of stress being there and have sometimes been sick, abused before they came to the shelter, etc. That doesn't mean it's not a good idea to rescue, places like Operation Kindness do a really good job observing animals to find out as much as they can about their disposition before adopting them out. But I adopted my Jack Russell Terrier from there and she was just the sweetest, most laid back and docile dog ever when I took her home. At her first vet visit we discovered that her spay incision was infected, and once the antibiotics started working, I had a dog that wanted to run around, play, bark, and seemed to be high strung with boundless energy. NOT what I thought I was getting but I was determined to stick it out. She calmed down after a couple of years and she's been a WONDERFUL pet. Just be prepared for the possibilities.
If you have a good feeling about this dog and are committed to adopting him, I say go for it but be prepared for anything to happen. Also, I'd highly recommend contacting Aaron Stewart at http://www.theeducateddog.com if you see any early behaviour problems that you can't seem to shake. This guy is amazing and his pricing is very reasonable-he helped us with our rat terrier in one visit. Can't say enough about him.
Best of luck to you!