Baby Baby Baby
Short hair does have less shrinkage than super long hair, but yes, it will shrink. There is a product called Biotin which can be found in your local drug stores vitamin section. Taken for 3 months, this will promote hair growth.
You work in a fast food place, if you are behind the counter, you could wear a hair net which is loose enough and would tame down any bizarre days you may have with your hair, we all have them in the beginning.
I understand you are concerned about you being a big girl not wanting short hair. But maybe you can look to the future of what you will look like in 3 years with natural, free form dreads. You will look beautiful. If you give in to extensions now, you would set back any progress that would be happening with natural dreads. Start with natural now, take biotin (it is not instant growth) and let those dreads start. People will not say "your face is rounder with short hair", they will say "look at those awesome dreads". Your hair will grow and one day those dreads will be down your back.
You can tie some beautiful bows with scarves etc to use as head bands to add a little oompf to the shortness when you go out. With dreads you need a vision to the future, not how they will look in the first year. Also I would like to say that Big Girls Are Beautiful,always remember that.