Baby food

My DD is almost 5 months old. She has suspected MSPI and is doing great on Puramino formula (her 6th one). She is finally eating and gaining weight! Our pedi told us we could start cereal at 5 months and to use Beechnut. This is the only advice he gave us. I am a nervous wreck to start! I'm not sure what to look for as far as reactions to baby food. With my son, I waited 5 days per food, then moved on to the next. With my DD, it took up to 2 weeks per formula to start screaming and refusing to eat. Her only other symptom is loose stool (no blood). My pedi just told me that in his experience, many MSPI babies do just fine on baby food and didnt seem concerned. We don't see the pedi GI doc til she's 6.5 months and I'd prefer not to wait that long to start food. Can anyone share their experience/timetable with baby food? Also....can anyone share reactions to look for and how long before they occurred? I'm worried because it took her awhile to reject formula changes.