on the one hand, i advocate letting your dogs work out their differences themselves, but of course you don't want your older dog to get hurt, or have a miserable existence with her new housemate. I'd suggest re-separating the dogs. make part of your house the new dog area, part the area for the older dog. let them sniff at the door to the room the other dog is in, for example, to keep them interested in the smell of the other. to socialize them to each other, find some neutral territory- like a friend's backyard or a park - somewhere that neither one of them can lay claim to or feel like they have the home advantage and they can just get to know each other a bit. remember that Maggie is not only having to adjust to a new dog, but also a new family, new surroundings, etc, and she may be a little extra defensive for a while until she settles in. another option would be to crate them where they can see and smell each other- but start with them across the room from each other, or at least several feet apart. gradually move them closer and closer together. keep one crated while the other is loose and as long as they are both non-aggressive toward each other, let the loose dog approach the crated dog- then switch. this will allow controlled meetings. after the dogs seem to be more or less okay with each other, you can let them meet outside the crates, but if they behave aggressively, back into the crates they go again. usually, under such conditions, your dogs will both adjust within a few days. if not, you may need to consult with a behavior specialist or consider rehoming your new dog, but these situations are rare- usually dogs will learn to at least tolerate each other if they don't fully get along. In general, if you can figure a way to convey to both dogs that you expect them to get along, they usually will.
I have a 10 year old chihuahua and a 6 month old EM and the chi is DEFINITELY in charge, but both dogs are ok with that heierarchy. and so are hubby and I. the only time we intervene is if the chi picks on the pup just because he's in a grumpy mood and she's having fun. sometimes if she's playing with a toy and his back hurts for example, he will just get in her face and yell at her for absolutely no good reason. and that sort of behavior i do not condone. but even if there is occasional grumpiness, i know nneither would hurt the other on purpose, and when to intervene to prevent Sadie (puppy) from accidentally hurting tyke (chi) with rambunctious puppy play.
it doesn't sound like your older dog has any of the grumpy issues that mine does, but it does help my dogs that i spend time with them together, try to involve them each in play, and also take time to spend with them individually.
hope that helps!
