My current dog was with us when we got a new pup for her. We made sure they we ok first. She loves him now and they sleep and play and hang together all day. Dog 1 is female age 3, dog 2 male 10 weeks.
I've been reading Cesar Millan books about dogs getting on. They should be fine if you do the right things. Make sure they understand YOU are the pack leader. His book "Be the pack Leader" opens with a story about 2 dogs that didn't like each other. Cesar sorted them out quickly and now they are fine.