If you post pics on the gender / breed forum, peeps will help you - https://www.backyardchickens.com/f/15/what-breed-or-gender-is-this
At two months old, it should be fairly easy to discern gender (unless they are silkies)
I'd say that your thoughts are correct. It can be a confusing time for chicks, when momma decides that she is through with looking after them. I can't say that i have observed a mother hen rejecting selected chicks, and continuing to care for others, but I'm pretty sure that other members will...