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.
pics would help.