Need some advice on adopting a older dog...

I'm considering adding another dog to our household. Although I swore off big dogs due to the heart break/health problems with my last girl. WELL- I've come across one on Petfinder http://www.petfinder.com/pet.cgi?action=2&pet=2588251&adTarget=468doggeneral&SessionID=4047741d51abd816-app2&display=&preview=1&row=0&tmpl= that has captured my heart and she's BIG. I even went and visited her today. She was very sweet and gentle. Her shelter tag says she gets along well with kids, and other pets. My number one concern is my children (5 & 8- both girls have grown up with dogs). My second concern is my little dog Molly. She's all of 12lbs but thinks she is about 1000 LOL... She rules the roost at the house. She is all alpha dog. She lords over Dat and both cats. Would it be possible to introduce a older dog into the mix? I don't want her to get hurt. When I foster, I only do puppies or very young dogs- so this has never come up.

Any advice? Too risky? I REALLY don't want a puppy. I do have my heart set on a older dog. These are the ones that get passed over time and time again. Thanks- Kristi