Hi there, Bev!
Since I have 2 pyrs and 2 little paws I thought I'd chime in here!
My little guys were here first - one is 5 years old and the other 3 years old. We adopted the pyrs last summer and discovered quickly that we were going to have to work out a system to keep them apart most of the day. And it sure wasn't because of the pyrs! My little terriers bark and nip constantly at them and it just gets really annoying! The pyrs are so sweet, though. They just look down at them with their tails wagging. They're not bothered at all by them!
Anyway, our system is simple. We use a baby gate to separate them. The little dogs get the kitchen, living room, and two bedrooms, and the pyrs get the laundry room, upstairs living room, and the two upstairs bedrooms. Also, the little ones are paper trained, so they don't have to be let out all the time. (the back door is through the laundry room - the pyrs' domain!) When we do let them out, they get the yard to themselves so they can run around without worrying about getting accidentally stomped on. We do let them play outside together sometimes, but only when we can watch them all closely so that nobody gets hurt.
I have a youtube video of them all playing together. I already posted it here a few months ago, but it shows how the little ones run the show at my house.
http://www.youtube.com/watch?v=JkrDJYZr_QYGood luck introducing your chihuahuas with your mom's pyr and lab-mix. By the way, I think you're wonderful for taking them in and keeping them together. It's such a kind and loving thing to do.
Amy