How to introduce a puppy to my current dog?

[Falling to hard, to fast]

I have a 4 year old male Australian Shepherd. We are considering adopting a 3-4 month old female beagle mix puppy. Our biggest concern is that they will not get along (in that case we would not get the puppy.)
For the most part our current dog does well with other dogs, but occasionally will get slightly territorial/protective (mostly with other males.) We want to give this the best possible chance of working out, so what is the best way to introduce the two dogs.
The puppy is already used to older dogs and larger ones as it comes from a foster home with a husky, a border collie, and a lab.
Thanks!

 

[CryCryCry]

I have always put the new dog in a room behind a closed door and let them sniff each other through the door for about an hour or so (but don't do this the other way around, because you don't want your current dog to feel like he/she is being punished.) Then, open the door and watch them with careful supervision.