definatly let the older dog assert its dominance over the pup that way they establish the pecking order between them from the start. If they are only mouthing at each other with some noise dont be alarmed just supervise their playing time as much as you can so if anything agressive happens you can take the appropiate action. My dogs are the other way around the older dominant dog is under 10kgs and my pup is 6months and he is towering over her although he weighs 24kgs, SHE is the dominant dog, she gets him on the ground in a dominant down(standing over the top of him while he is in a submissive position). Thats the way THE DOGS sorted it out form day dot.It is best to let them sort it out as long as your comfortable with how rough the dogs are playing then its best to leave it to them