The dark faces of this variety cover red, so it is more difficult to tell the gender. You're looking at comb size alone.
They do look small for 12 weeks old. Age is important when judging comb size. But for now, I think they all look to be pullets.
Purebred blue Plymouth rocks are very rare, but the name blue rock is often used for the hybrid also known as Sapphire Gems. That's my bet just because the cockerel doesn't seem large enough to be part Jersey Giant.
But I agree with the above post. There is no way to be certain.
She's a Dixie Rainbow, now often called simply 'Rainbows'. It's a fast growing hatchery hybrid raised to be good for meat or as an egg producer.
The breeds used to make them are unknown but I would guess meat strains of Delawares and possibly red rangers are used. These won't lay blue or green...
It's helpful to know the original source of the bird. Hatchery, store, backyard breeder? Without that info it is a guessing game.
She could well be a mix. I think you are right that she is not a purebred cuckoo Marans. Too large, feathers too loose and fluffy.
Barred rock is possible--...
The first two are cockerels, the last a pullet. I would show the last two as Rhode Island red bantams, one rose comb RIR cockerel and one single comb RIR pullet. They have more of the Wyandotte body type, though, but at least the coloring is good for Rhode Island red.
The first cockerel doesn't...
From what I can tell, the comb isn't that large. It would help to have a clear photo of it, which means moving the crest feathers out of the way. I'm going to say female for now based on the lack of a crow plus the comb. But it is often iffy with Silkies.
Production blues are Rhode Island Red crossed with blue Australorp, but neither of these breeds has white ear lobes. I think he is a different hybrid. Maybe Starlight Green egger? Handsome guy either way.
Yes, probably a Leghorn mix but white Bresse is also a possibility if the legs are slate instead of green. The white beak makes think that could be the case.