We select for good temperament first, no human aggression, not nasty to flockmates. No physical defects, equally important. Then correct structure and type for the breed, according to the SOP. If you aren't selecting for show, more flexability there if necessary.
Removing cockerels is a...
Mine are from Cackle, and I do like them. You might find a cockerel from a breeder, but be very careful doing that, you want to avoid introducing disease into your flock. And, where are your birds from, is that a possible source?
Getting chicks in spring would be my choice, straight run, so...
I agree. For egg production, use another leghorn! White production type leghorns produce best, but sticking with brown would keep your flock producing well, and breeding the same genetics that you already have. I do like the browns, have some myself.
Mary