Congrats on your new little one
As a former owner of a Lhasa
I think that you'll have no problem with a new puppy
I've found that Lhasa's aren't as rambunctious as say shih tsu's
and generally older dogs realize that they are dealing with a puppy
when they play
The tricky part is getting your dog to accept this new little guy
because he is going to be stealing the spotlight
Best of luck and enjoy