12 year old dog and new puppy

Hello everyone!
 
I am an alumnus of the old site and a huge Sarah fan!! I took Comet, my GSD/Golden mix, to her NH training program. Well, Comet is now 12+ and slowing down immensely. A young lab mix (not sure of breed)  was found as a stray at my husband's police station; he is housebroken, adult teeth but still a puppy. We just brought him home today and have a vet appt for tomorrow. He is not neutered but will be as SOON as possible. I am not a big puppy/housebreaking fan so I was happy for a chance to give an older pup a  home.
 
My Q is: he has only been here since the afternoon...the dogs did fine for a while, Trigger wants to play MUCH more than Comet but Comet was tolerating it and just walking away. Now, though, Comet has started growling and lunging aggresively at Trigger without provocation...Trigger can't even walk past him.
 
I told my husband I did NOT want to stress my geriatric dog out at all by adding a new dog. but I am wondering if this is something they will work out?? How should I be handling it? ignore? Praise one or the other for certain behavior? Trigger seems very scared of Comet now and I feel awful for both.
ANy help appreciated, thank you so much
 
Deb