I am not one to normally comment, actually I have never commented on anything like this before, but your question hits close to home for me. I have a daughter that will be 3 in November. She currently weighs 22 lbs and wears size 18 month clothes. She has always been small and we have always supplemented her diet. Her main source of nutrition right now is pedia-sure.
We spent the first two years of her life going to doctor after doctor trying to figure out what to do. We have had genetic testing done, chromosome analysis, swallow studies, GI scans, MRI's, you name it, we have probably done it. For awhile we were seeing a group of specialist at our local children's hospital. They tossed around the idea of a feeding tube with us, but decided against it. Finally it was decided that she was probably just small. Her pediatrician keeps a close eye on her and as long as she continues to gain weight they say she will be fine. (I was told that as long as they are gaining weight even if it is small amounts, they are not considered failure to thrive).
I should also add that she was born with hearing loss. Her ABR's had her hearing at 80 decibels when she was born. For some unknown reason her hearing has started getting better. She no longer wears hearing aides, but we still do have some hearing issues. She has also had tubes in her ears twice now.
She definitely has a few struggles, speech being one of them, due to the hearing impairment. She has a speech therapist and an eating specialist that come into our home and work with her a few times a month. We do regular visits with our pediatrician, which is normally just a weight check to make sure she has grown at least a little. We have just accepted the fact that she will always be tiny, and that is okay, because there are worst things in the world. In every other way she appears to be healthy.
I know you have a hard decision, and as a parent, I think you just need to choose what feels right to you. If you do not feel right about a feeding tube, maybe it is time for a second opinion. If your daughter is progressing well mentally and physically (motor skills), then maybe it is that your daughter will be small. If you feel like she is truly struggling and not getting the nourishment she needs for proper brain development then maybe a feeding tube is the way to go.
I don't know how truly similar our stories are, but just from the information you put in your question, it almost could have described my daughter. I hope that hearing from another parent that has been through similar situations will help. Good luck!