Help, don't know how I'm going to be assertive with my parenting decisions.

So, I am a ftm. I'm 19 years old and having my baby girl in January. I am a researcher, and have researched extensively everything that I want to do for my child (cosleeping, breastfeeding, kinds of products and foods I want my child to be exposed to, etc). My SO mom and grandma love to give me advice and boss me around when it comes to things for my baby. My SO's mom hasn't had a baby in 15 years, and my SO's grandma hasn't had one in 40! They still think they know everything that is best for my child. I love them like my own family, but I don't think that they will respect or follow our choices when our baby is born. I know what is best for my baby, how do I express that to them without being rude? I don't want to hurt their feelings, but I have done so much research and read so much I know what I'm doing. These are my decisions to make, I know that, but I don't want to make them upset at me, and I want them to follow my parenting style when they babysit. Please tell me I'm not the only one struggling with this!!