Yes, medicated chick starter is designed for any chick from hatch to 18 weeks old, so it is fine to feed at 0-2 weeks and 11-12 weeks of age.
Is this feed store set up where you go get the bag yourself and take it to the checkout, or where you pay for it sight unseen at checkout and someone brings it to your car from the warehouse in the back? If you didn't pick it up yourself, and if you don't want to use it, and you didn't ask for it, then a well-run feed store should exchange it, or allow you to return it for in-store credit, or a refund. I have had two incidents where I have even asked for the wrong food (so the mistake was totally my fault) and my feed store has exchanged an unopened bag. I have had a few incidents where I bought food and wasn't happy with it, and the store allowed me to return it even though it was opened, which I thought was exceptional. Have you asked to speak to the manager? The employees often don't have the authority to do refunds or exchanges, but managers are usually more cooperative. There is absolutely no reason for a feed store to be unwilling to exchange an unopened bag that was purchased within the last 2 weeks, or perhaps even a little longer depending on the manufacture date. Do you have an alternate feed store to use? I have at least 10 within reasonable driving distance, so maybe that's why my store has such great customer service.
Regarding your 11 week old bantam, she could be just trying to make friends, or trying to mother them, or wanting to eat them, or just trying to get away from someone that she fights with constantly. There's really no way to tell from what you've described. If she's housed with someone that's making her miserable, she might do better being moved to another pen, that is if they're actually fighting and not just playing. Constant fighting is very stressful to anyone, and is more than just establishing of a pecking order.