What is her BMI - that is more important than her stat on the growth chart. I would suggest to keep breastfeeding or keep giving formula until 2.
My daughter is dx'ed failure to thrive. My husband and I are both little and right now she is 3% on the growth chart (20 months old, 20.12lbs and 32inches tall) but she eats everything.
Avocado is a very fattening and healthy food - mix it with some banana or infant cereal or somehting and its great. Also try yobaby yogart (mix it with the avocado or baby cereal).
For kids/adults with CF having a good weight can mean the difference in fighting on infection before you even need IV's or being sick for a month. I got down to 97lbs this year and I was really really really sick. I got my gtube in august and its the best thing I ever did. My weight is now 114 (5'3) and I am feeling great b/c of it. I know that if I get sick and don't wanna eat that I won't suffer b/c of it. Its definitely lowered my stress factor in regards to my health.
I think that you have to do what feel right for you. If you think she is just tiny, is following her own growth curve, and eats well(and isn't sick) then for now you can probably hold off. But don't think of needing a gtube as something bad. Think of it as a step in the right direction for great health.
-O have you looked into a different brand of enzymes? I know that sometimes this can make a big difference with weight gain.