Hi,
You said that she does not seem to get full on formula alone. If she's hungry, she'll still take the bottle. If she stops sucking on the bottle at a feeding, she's full. Babies instinctively eat until full and automatically stop when full.
The saying "sleep like a baby" is just not what we would LIKE it to be. Babies rarely sleep more than 4 - 5 five hours at a time.
Anyone who gave their baby cereal before 4 months by the dr orders obviously had special situations such as vomiting issues... and then, had special orders of when/how.
Of course, you could try a spoon of cereal in her bottle... but I would suggest doing it in the daytime so you can watch her reaction, not at night when you might sleep through a choking or something.
I assume the lady who cut the nipple with toenail clippers sterilized the nipple afterwards.... ick! LOL
Bottom line, try what you think might help. It may work for you, it may not. Dont let others pressure you to do it their way. But remember: these scientific and nutritional studies were not done haphazardly. Take them very seriously.
If your baby is healthy and thriving, I'd wait on the cereal in the bottle. She'll eat until she's full. What is she doing that makes you think she's not full after a feeding? just curious.
Good luck!!
M.