It does not sound like your daughter is too small. She seems to be steadily growing. She is a pound smaller at every stage that my dd, and my dd is above average on the scale.
As far as the whole solids thing goes, here is my opinion. You have to do what feels right for your baby. Some people say your horrible if you start before 6 months. Others say you can safely bf exclusivley for a year. Everyone else disagrees on those 2 points. I was planning to bf exclusivley for 6 months, due to the increased risk on allergies if you start sooner. My dd started getting really upset at night when my supply was at its lowest. I pushed on for 2 weeks, hoping her increased demands would up my supply. It helped a little, but I decided to give her rice cereal in the early evenings at 5 months old. She ate the whole bowl like a pro the first time. And I felt so guilty.
Just last week I was sent an article from Baby Center stating that new studies show that if cereal or some other solids are introduced BEFORE 6 months you could be LOWERING their chances of food allergies. Well, make up your mind!!! That is why you have to make your own decision, and know your own baby. My daughter was clearly ready. Yours is probably not. They say if there is a problem introducing solids, stop for a couple of weeks and try again. Make sure you feel calm and happy when you try again. She can feel your stress and may act accordingly. Smile and say mmmmmm... when she takes a bite. Good luck!