C.G.
Great job breastfeeding this long. Keep in mind that supplementing will probably increase the baby's reflux issues unless the reflux is due to you having a milk overuspply/ forceful letdown.
At any rate, it seems like most babies take better to formula when it starts out mixed with breastmilk. You can start wuth 50/50 and then gradually decrease the amount of breastmilk per bottle. I always found my kids took well to Good Start when I gave them formula.
You might expect her BM's to become a little more stinky- but maybe not if you are just going to supplement a little bit. It's possible that she could experience some constipation, but again if you just supplement a little bit it may not be an issue.
Another thing to kepe in mind is that some docs have their breastfed babies on iron/vitamin drops- you want to make sure baby does not get too much iron from the combination of vitamin drops and formula.
Good luck!