Starting Solids!

I have an 18 week old baby who can hold her head steady and sit very well with minimal support. She's a big baby, 97th percentile, and exclusively breastfed. My pediatrician said she recommends introducing solids around 5 months, but I know so many people start at 4 months. Is it okay to slowly introduce a bite here and there of pureed fruits or vegetables? Do you have to start with rice cereals?