My pitbull/corso mix 3month old puppy plays too rough with my 3yr old chihuahua.?

My fiance and I adopted a male chihuahua last summer from a rescue. He is people friendly but is wary around children. He is friendly with female dogs but can be aggressive with male ones. We recently adopted a female 3 month old pitbull/cane corso mix. He seems scared of her especially when she plays with him. She plays rough and mouths occasionally. Several times they will play and she will nip his behind and he yelps. He doesn't seem to enjoy playing with her. We have not been able to walk them outside together because she was sick and we are also waiting to get her second set of shots. How can we get her to stop playing so rough? I know she is just a puppy but I don't want this to be an issue where the little guy can get hurt later. Thanks (Even when he is drinking water and she walks over to drink he stops drinking and moves away.)