Silveruuds should never have crests, neither males nor females, so I wouldn't trust anything this breeder says. In fact no breed of chicken has females with crests and males without them.
The red and white one is a mixed breed cockerel. The black and white one is probably a pullet, and is also...
He does look like a Deathlayer mix, and it would have to be with something with feathered legs, probably Silkie. I believe the gene for 5 toes is dominant, but the Silkie parent itself could have been mixed, and that may explain why he only has the four toes. Cochin or Marans are possible, too...
Your older hens are all mixed breeds.
It's actually Melly that looks more like a male to me. That comb is very large for 8 weeks.
Easter is kind of in between, but looks more like a pullet at this point.
The first is likely to be a crele or spangled old English game bantam.
The second is probably a red Cochin bantam.
The last two could be wheaten Marans. They lack the beards of Faverolles and the pea combs of Brahmas.
Looks more like a lavender Orpington to me, but if you look at the skin on the underside of its feet, that should tell you more definitively. Orpingtons should have a white or pinkish white color to that area, while SGs will have a yellowish color.
Difficult to say for sure, but that could well be correct. Jersey Giants get big but they take a long time to get there, so the black one could be one. Mystic Onyx should have dark face and skin, but there's always the oddball that doesn't. Or it could be a legbar x black copper Marans type of...
At least here in the US, auctions are well known for these problems, and you risk bringing in disease to your flock by buying from them. I mean, that is always a risk, but more so from auction birds. If I were you I would seek out a breeder with a good reputation. That's the surest way to get...
The ethical part comes in with how you advertise them if you sell them. There is nothing wrong with selling mixed color Cochin bantams so long as it is stated to potential buyers. The trouble comes from people who sell mixed birds as purebreds. That is false advertising and it's unethical...
It's not that uncommon to get chicks with unexpected coloring, especially with mixed breeds like black sex links. It's just the fluke of genetics that sometimes non-dominant genes from the mother mixed with non-dominant genes from the father combine to express those hidden genes in an offspring...