Hi N.,
take a deep breath, remember you're a great mom and babies are designed to stress us out.
Next - someone has to be at the 1st percentile or it wouldn't be the 1st percentile. More important then where she is on the chart is the big picture of what she is doing in general when it comes to growth (including length and head circumference) and development.
Cereal in the formula or solid foods are unlikely to increase her caloric intake unless it is on top of what she currently takes rather then instead of some of what she currently takes.
My son started at 55% for weight and headed straight to 3%. He also had severe reflux, recurrent yeast infections and a couple of other things to deal with. Our wonderful and mellow pediatrician ruled out the big malabsorption issues such as cystic fibrosis and then said "ok, this is him". He was gaining length well and head circumference (always had a big head) but was slow to gain weight.
He was also (still is) a very active child always wiggling and squirming. It doesn't sound like much but I work with babies a lot. It makes a difference in weight gain if a baby is quiet and calm, rarely moving much versus always active. The active babies burn more calories so gain weight slower.
You may want to try human milk. Relactating can be a lot of hard work, however there is the option of donor milk through the San Jose Milk Bank. You need a prescription and it is pricey, however it may be the boost your daughter needs. Often donor milk has higher calories per ounce. They also sometimes have milk that is clear of some of the more common major allergens, you just need to check with them (if that is an issue for you). You can also add powdered formula to the human milk to further increase the calories though this needs to be done with care. Increasing the concentration of formula can be dangerous as it can mess with electrolyte balance and cause other problems. It should be planned by a pediatrician or pediatric nutritionist.
If your 6 month old daughter is growing and developing fine except for weight, then I would not be overly concerned. Asking to check thyroid and possibly do a sweat test to rule out cystic fibrosis may be reasurring though.
Our son was 18 months before we could turn his car seat to face forward (20 pounds) and at 8 he is almost to the 10th percentile for weight (and that due to muscle gain). He is a healthy active boy who is bright and actively participating in life at many levels.
The reflux will usually be "grown out of" though may reappear later in life. The more severe then often the longer it takes for the child to learn to control it or for the valve to mature and strengthen. Certainly starting solid foods before your daughter has the oral development and general readiness is not likely to help.
It does sound like an evaluation by a pediatrician would be advisable.
I hope you find the support you need. Feel free to write directly is you wish to discuss this further. If you are on the Peninsula or South Bay I may be able to recommend some peds.
K.