If she's broody then it makes sense. Many times I have had broody hens sit on eggs and raise babies together.
If she isn't broody then I would keep her away.
It is a myth. It is true that many European breeds with white earlobes lay white eggs, but this is something that has probably been selected for. White eggs were preferred in the past to brown or tinted eggs, especially in European countries.
Breeds like the barred Holland were bred to be dual...
The mother is a lot more brown than I would expect for Bielefelder x lavender Araucana.
I think the chick will be darker than her but will still have some brown. It will definitely be barred, like the Malines father.
The RIR and black olive egger look to be males.
The Rainbow is king of in between. I think it is probably a pullet.
The brown olive egger and the Wyandottes look to be females.
Leghorns have big combs in general, but usually they are upright in males. Not always though. Hopefully it won't be a problem for him. Polish chickens cope with their enormous crests, so I'm sure he will adjust. I don't know why he would toss it around. Probably just something he enjoys.
Polish are very difficult to tell gender, and sometimes it takes until they are several months old. But to my eyes, I do think you're right about the bigger one being a pullet. The crest is tidier than a male's crest would usually be.
I'm not sure about the smaller one. In the coming weeks...