My Jack Russell, Domino, is a Pet store puppy (PetLand, she was 9 weeks when we got her, and had been in the store for just a day or two), and her skin gets irritated sometimes. But the vet's say they're probably just bug bites and she irritates them further by licking and scratching at them.
We started taking her to the dog parks after she got her shots to socialize her and get her used to other dogs. She loves to play and wrestle with my parents Golden Retriever. It's funny watching the 60-70 pound Golden play tug with my 17 pound Jack Russell. It's a fairly one sided game, especially with tile floors.
Domino was a little ankle biter at first, but we were able to nip that habit with a little trick I read on here and some other sites...
If she's being nippy, say Ouch! in a high pitched yelpy voice, hide your hands from her and ignore her. If she bites at your arms, yelp again, get up and stop giving her attention. She'll learn that biting means playtime is over.
We've gotten Domino to stop for the most part, if we're playing rough, sometimes she'll put her mouth on my arm or hand, but she won't apply any pressure. If she does, it's game over. I was doubtful that the trick would work, and it took a while for her to figure out, but if you keep it consistent the pup will get the idea. Good Luck!
Thanks!
-Sean