Kairo is 8.5 months, and Sugar is a 10 year old Bichon/Poodle. Before Kairo got to be 5 months, they got along fine and would play together and even take naps together in Kairo's pen. Then around 5 months old, Kairo just got too big for Sugar, and even though Kairo would play like he always he had, he accidently knocked Sugar backwards while playing, Sugar landed wrong on her tail and sprained it.. A few laser treatments and several hundred dollars later, and we learned the hard way. They are now separated most of the time except for when we let them outside in the fenced area where they have more room to run around and are supervised. Sugar is no longer afraid of him, but at 10 years old, she has a low tolerance for Kai's antics, so when she is done playing, then she is done playing, and she'll let Kairo know in no uncertain terms lol.
When we are inside, I generally only let them interact together when Kairo comes in from exercise or training, and he is already exhausted. He generally ignores Sugar then, but only til he catches his second wind, at which time, they are separated again. But with Sugar being older, she's content to lay in Kairo's pen with the door closed so Kairo can't mess with her, while Kairo lays on the vent in the foyer when we are all together watching tv. Kairo has always been a high energy puppy, so I am hoping he will settle some as he gets a little older.