You can probably stop feeding puppy food around 6 mo. they aren't growing as fast and don't need to have all of the extra calories. If she is already eating out of the other dogs bowl just slowly start decreasing the amount of puppy food you give her and increasing the amount of adult dog food fed in the other bowl.
Most small dogs go into heat between 4-10 mo. with the average probably being around 6 mo.
Good luck on keeping your boy away. If you read the breeders section you will see that dogs can be pretty sneaky. And even the most unexpernienced of doggy men can get the panties off your little girl. Maybe we should devise a puppy chasity belt.
__________________ Anna  & Roxy  |