Good luck, it's tough sometimes introducing a younger dog into an already established 'pack'....I wish I could give you some help. We had something similiar...a 12 year old lab, an 8 yr old lab/beagle/spaniel mix and then we brought in a year old rat terrier....the middle dog was indifferent and would just ignore the rat terrier....the lab hated the rat terrier and he could barely enter the room without my lab going after him. They would at times all lay together snuggled up and then a half hour later the lab was after the ratty. The middle dog would just go into his bed and ignore everyone. At times it was peaceful but at times it was not....I could never leave all 3 together without supervision. About 8 months after getting the ratty, I had to have my beloved lab put to rest (just old and his body gave out)...for 2 weeks my middle dog moped around, very sad....mourning I suppose....and then one day he became best buddies with the rat terrier and now they are inseperable like the lab and mix used to be....after that experience I am not sure I will ever introduce another dog under those circumstancez...I know I didn't do things right, I should of waited until the lab was gone. Ultimately, I did get the rat terrier because I didn't want the mix to be alone when he was gone, I think I should of waited....I have alot of guilt about disrupting my aging lab mostly, leaving the mix so confused he would just go in his bed, and the poor rat terrier who I'm sure was wondering if anyone truley loved him. All is great now, but I think back and wonder if I should of waited....I wish you the best of luck....my situation has weighed on me for over 2 years, Brutus has been gone 3 years this November. I am sure there is help out there to help make your situation perfect, hope someone can help you out.