I have a 14 week old chihuahua mix puppy.  He is very energetic, and tends to be quite nippy.  He is getting better about not biting me, although he tends to nip people he is not used to.  He also tends to bite other dogs that he meets.  I recently moved in with a roommate, and she has a 2 year old female hound mix.  My problem is that my puppy will not leave the other dog alone.  He constantly chases her around biting at her face and legs.  She usually puts up with him, and play wrestles with him sometimes, but after a while she tends to get annoyed and growls and is slightly aggressive towards him.  I am worried that she will lose her patience at some point and end up hurting my puppy.  I always watch them when they are playing and try to stop them if they are getting too rough, but I am worried that since my puppy is so much smaller, one wrong move could be disastrous.  Should I allow them to continue playing like this so that the puppy can learn his place, or is this too dangerous and should I keep them apart until he is slightly older?  Any advice would be greatly appreciated. 

I woulnd't keep them apart - your puppy has a great opportunity to socialise with your friends dog.  BUT I wouldn't allow them to play until one gets so fed up you fear physical damage. YOU should call a halt to the game before it gets too much, and then seperate them.  In my experience, dogs who are allowed to play like yours soon learn to use their teeth to get what they want, from other dogs and humans.

I'd also be concerned about nipping people he doesn't know - what do you do when this happens?

When he nips other people, it almost seems like he's testing the boundaries.  With me, he knows that he can't get away with it, but when he meets someone new he seems to see what he can get away with.  He doesn't bite strangers when we are on walks or anything like that, but if I have a friend over who is playing with him, then he seems to get nippy.  I usually have them do the same thing I do when he nips, which is to give a little 'yelp' if he bites too hard, and then if he keeps doing it, to just get up and walk away.  It seems to have worked with me, he now knows not to bite hard or the game will be over.  But with new people, it doesn't seem to always hold up.  Also, I think part of it is that he just gets very excited when someone new comes over, and sort of forgets the rules.  Is there anything else I could be doing to make sure he remembers his training even when he's very excited?