When I got my late Pit Mix, Gus, my 2 older dogs were introduced to him at the vet and they were allowed to get to know each other for over an hour, to the point all three of them were wiped out. Then we all got into the car, and went home, no problems. After my lab Joe got some sleep, he did a whole lot of glaring at Gus, but there weren't any actual attacks, just a lot of dirty looks and gulping when I said something to him about it. For the first 4 days, Joe barely tolerated him. My other dog barely seemed to know he was there. On the 5th day, at the vet's of all places, Joe suddenly started playing with Gus. They were very close, with no real problems, until a series of TIA's and strokes made Joe kind of lose his mind and attack him. When Gus was 6 months old, he was much stronger than Joe already, and when fully grown, at 75 pounds, Gus was actually very gentle when he finally defended himself. Joe couldn't seem to accept losing and went downhill overnight and we had to put him down.
I would do it now, just to keep their ages as close as possible.