Second Dog in Home. In over our heads?

Hello,

We recently took in a second dog as a foster on a temporary basis while deciding whether to adopt her permanently. We're running into some aggression issues that we don't really understand.

Here is some background about the dogs. The first is a neutered male beagle/hound mix and weighs 30 lbs. He has had his run of the house previously, but is kept out of the bedroom and bathroom areas. He also has no tail. The new dog is a spayed female beagle/collie mix and weighs 30 lbs.

We met in neutral territory, in a park that neither had been to before. They seemed to get along fine - although she seemed a little shy, they nuzzled and pawed each other a bit and never showed any signs of aggression for the hour we were there. We agreed to take her on a foster basis for a week while we determined if they would be a match.

Since we first met her, we noticed she's a bit skittish. Part of this may be that she's used to living in the country and now she's in a more urban setting. However, she seems generally fearful - doesn't like to go through doors, sometimes will just freeze in her tracks, often has her tail tucked unexplainedly.

Once we brought her home, she became aggressive/dominant(?) towards the male dog, doing things like guarding him out of the room, growling at him, taking his toys while he was playing with them, etc. They don't seem interested in playing with each other. It's a bit confusing to us that she would be the dominant dog because she's so fearful. We don't know why she's acting aggressive. We're a bit scared that she might try to attack us or turn her aggression on us next, but right now she seems very protective of us and tries to "guard" our old dog away from us.

To make things worse, when we're playing outside, he's dominant over her and she acts very submissive.

Today is our first full day with her, so maybe she's just settling in. I don't know though. We both feel like maybe we got into something we shouldn't have and aren't sure where to go from here. We expected all the normal training issues and some kind of dominance issues, but we can't even tell which dog is dominant.

Please help. Any help would be greatly appreciated.