Neutral territory is very important here. I wouldn't introduce them without a barrier at either your home or the foster home. They need to meet in a place that neither of them feels protective about. it should eliminate any territorial issues. They should both be kept on leash with the leash held by someone they trust.

Stay calm. If you appear nervous or raise your voice, Merlin will assume that he should be upset about this new dog and bark more. "If Mom's upset, I need to protect her!"

Treats, lots of treats. If Merlin is quiet for a split second, he gets a good doggie and some high value treat--something he loves and doesn't normally get alot of. I use strong cheddar cheese for most of mine. They get other treats at different times, but nothing gets their attention like stinky cheese so I use it when they need to learn something new.

When Jean-Luc comes home, make sure that all the dogs have a safe, special place just for them---a bed, a crate, something where no one else can bug them and where they can take a special toy or treasure and not worry about it getting stolen.

Raised hackles dont neccessarily mean aggression, they could just mean the dog is a little freaked out. Watch all their body language--what's his tail doing? A relaxed happy wag or sticking out behind him and wagging? Turning his head away is a good sign. Dogs don't make direct eye contact very often, unless they mean business.

Since it looks like you are going to be a multi-dog household soon, I'd recommend checking out a couple little booklets both by Patricia McConnell--one is called "How to be Leader of the Pack and have your dog love you for it" . The other is "Feeling Outnumbered, How to manage and enjoy your multi-dog household." I love them both!

Good luck, Have fun.

PS: they are both neutered right? That will help reduce problems too, especially as Merlin matures.