We were all new once. Btw .
Explore the site, there are tons of things to learn about on here.
As for the tail feathers, some grow them fast and some slow. My 1 week old mixed chicks have tails already. But my chicks last year took forever to grow theirs.
So these chicks didn't come from a feed store, but an individual? Do you know if they were breeding and have parents on site or if they ordered hatchery chicks? Wouldn't hurt to call back and ask if you don't know.
For me, the ones that have NO tails for a while will ALWAYS be boys! Doesn't mean the one with the tail won't be a boy. But seriously, no tail... ALWAYS boy in my experience. Early tail, irrelevant.
The spot on the head is not how hatcheries sex their chicks. They do it by vent. The spot can be very deceptive. I had a girl with a large white spot who I worried about for the longest time because of that reason. And then as they feather in the boys will have double wide white bars. So the females will appear darker in color than males. Essentially black with white stripes. The males will appear as white with black stripes. It is a significant difference, just google up some images and you will see what I mean. However, hatcheries don't have birds that fit the standard of perfection to a T and many back yard breeders don't care enough about the the standard to cull parent stock that doesn't fit. So there can be variations that keep you guessing for a while. Some barred rock can be sexed at hatch by spots, I think. But those may be show quality. Most barred rocks will have a head spot, just of different sizes. If mated to a solid dark color male, a barred rock hen will put off sex link chicks where only the males will get the white spot on the head... because males get a double dose of the barring gene from their mother. Hence the double wide white stripes on cockerel.
Even birds that do get vent sexed have mistakes happen. I've gotten several boys that were supposed to be pullets on different occasion, from different feed stores, that use different hatchery sources. It happens!
Give it a little while, until the stripes come in.. too early right now.
Thanks for the really detailed reply, it's not the end of the world of on is a rooser. The person we got them from showed us the difference between male and female so I'm hoping they are female.