Not sure how much help I'll be:
With that much difference in size I'd keep an eye on them when they are together. When we adopted Kaiser we were still fostering a 5lbs chiweenie puppy. They loved each other but even without actually being rough a gentle pawing from an 80lbs puppy can be a bit much. We kept a constant eye on them and would calm them down if their playing got too enthusiastic.
When we left the house both puppies were crated. Finely was still being house broken and we didn't have a good gauge yet on Kaiser.