New puppy (large breed)and adult dog (small breed) not getting along, tips and success stories?

Hi,

I'm looking for tips or success stories of bringing a new puppy into a home with an established dog. Especially homes with a large breed and a small breed, and especially if they're both male.

We brought home a new 8 week old large breed puppy last week and already have a 3 year old small breed dog. They are both male.

The puppy is not respecting the adult dogs boundaries (jumping on him, nipping him etc as puppies do) and the adult dog became very upset and distressed about this. I think its scary for him because the puppy is already larger than him.

We do step in and don't allow the puppy to pester the adult dog, but now even if the puppy even comes close, the adult dog gets anxious and sometimes barks as he has developed negative associations of the puppy.

We tried to follow all the proper techniques and advice for their introduction but things haven't gone how we had hoped. I feel worried about how we can fix their relationship.

We're going to see a behaviorist soon but also wanted to hear of anyone else's success stories, so I can feel a bit more positive about the situation.

We have a very small house with no garden, so there isn't much personal space for the dogs. However, they're crate trained and we bought a gate to separate the downstairs (downstairs is just a single room).

Thanks