Did you get them from a hatchery or feed store? If so, they are likely a bit of a mixed breed, often also called "production reds." One mix used is RIR and New Hampshire Red. Of course this will cause some variation as you describe.
I usually make my best gender guesses by stance -- roos are more upright -- or slower feathering in roos. Or, if you're lucky, the little roos will start trying to crow, even as young as a few days. Of course it doesn't sound like a crow before around 12 weeks, but they do stretch their necks out and make a different sort of squawk.
Happiness is when you can look at your chickens and smile....
Worry is interest paid on trouble before it comes due.
Flockwatching my little bunch of mutts, a favorite pastime.
pics would help.