I need some advice. My pediatrician told me today to start giving Olive and Grant rice cereal. They are 20 weeks old. She said parents who wait too long have children with "texture issues."

Once I get into a routine, is it best to give them breast milk and then cereal or cereal and then breast milk? I don't want them to be so full from milk that they won't eat cereal but I don't want them to be so hungry they won't eat from a spoon.
Here's another question. I need to take medication for pain that I can't take while breast feeding. The babies refuse to take a bottle, even with breast milk in it instead of formula. I have tried everything I can think of from having someone else do it to different nipples to different times of day. I have been told when they get hungry enough they will take a bottle but I don't feel comfortable pushing it past a certain point. My only hope, I think, is when the time comes for them to take their liquids from a cup. Any suggestions on how/when to make a smooth transition to cups?
Sorry about the questions. I have a 14 year old and a 9 year old so you would think I would have this figured out. It just seems more complicated with two......