Kitten and dog

We have a löwchen, 4 year old, called Stella and a 10 year old Siberian cat called Stitch. We’re getting a new kitten. Mostly because the dog loves to play with our cat, but the cat has gotten arthritis and is getting old. The cat has a cat door that is chip regulated, so he walks freely all over the house. The dog can stay on one floor. We were thinking to do the same with the kitten, so the old cat can get some peace and quiet.

But; Is it safer to get a 12 weeks old kitten? Or will a 6 month old bond the same way?

Will the dog be more accepting with a small/young kitten? Than a teenager?

What’s your experience?