My guy is about 8.5 pounds and since my other 3 dogs between are 30-40 pounds, that still seems small to me!
Don't worry if they do a little fussing at first. They just have to establish their pack order and set their boundries. Once they figure that out they'll be fine. Just keep an eye on them first (especially the little one!) until you're sure how they are together.
Before you know it you'll have one big happy family!
The bad breath isn't usual. Sounds like it might be dental problems... You might try to do a search through the older threads here. I'm sure you'll find some good advice.