Should we go and adopt a dog?

The Humane Society in my area is selling dogs for $17.76 (sorta like a 4th of july sale) and they have a German Shepherd there (a rarity in my area) that my family wants to adopt. Then there is another shelter that has fallen on hard times I guess and their dogs are free. They have a Rottweiler (again a rarity) that my family also wants to adopt. BUT our pit bull is pregnant, she is only due to have 5 puppies though. There's no telling how long these dogs will be available so if we want one of these dogs we have to decide, well now. Do you think it'll be alright to buy another dog while our current dog is pregnant? And yes she will be spayed when the puppies are gone.