I'd contact them and ask. 6 weeks is a very early cutoff. But, if that's their policy, I agree with Jed. I'd choose one with a very typical female pattern to be sure. And it's a pity because the unusual ones can be the most stunning.
6 weeks?! That's a bit soon. The patterns will usually tell the tale even in juvenile plumage but I can't see enough of it yet to be sure of anything. Post once the feathers fill in.