You're not going to be able to just put your new chicken in with the others anyway because they'll need to be integrated slowly. Otherwise your older birds are likely to kill or seriously injure the newcomer. You'll have to rig up some kind of enclosure for her within sight and sound of the others for a bit until they get used to each other, preferably right in the coop. One of the ways to speed that integration up is to put the new girl in her "introduction" pen or cage with her own food and water and leave her for the first day. The next day you can spread some scratch right at the border of the older girl's pen and the new pen. Just spread a line of it on both sides. The older girls will begin to eat the scratch on their side and the younger one will eat the scratch on her side, effectively having them eating side by side without being able to hurt one another. After a week or more, depending on how that goes, you can try to open the youngter's cage and see how it goes - supervised, of course!
@cavemanrich
is right about what to feed. Starter grower or an All Flock means that they will all be eating the same thing. This is what I do when I have a mixed flock of young, older, and roos. Good luck!
