I started mine at just under 5 mos. That was 2 yrs ago and I have also been hearing more and more to wait til 6 mos. I actually looked up some info on it the other day to give to a mom who was starting her baby at 7 wks, and what I read was that the "open gut" (lining of the intestines not being formed yet) goes away sometime between 4 and 6 mos. So I guess that is why they are leaning towards 6 mos now. I don't know how I would have made my son wait another 5 or 6 weeks though. I didn't start because I had been given the go ahead or had a plan to start on a certain date, I started when all of a sudden he was ravenous and it wasn't just another growth spurt with cluster nursing. I gave him cereal and he didnt even spill any. So I guess I will just hope my next one doesn't do that until after 6 mos.
You could ask your ped if there is any way to know for sure when the gut isn't open anymore. Maybe the signs of readiness are indications of that, I dont know.
And always bottle first. She should get her nutrition from her formula or breastmilk still primarily. Cereal and solids should come second. I was told to do the nursing, then about 20 mins later do the cereal/solid. So basically just enough time to make it and get him settled in. Any longer between and it will stretch out her feedings too far and she'll be taking less formula.