Definitely not ready to me... I have been told 80% brown fibres, so wait until most of the white fibres are brown otherwise you will miss out on yield.
I am wondering if you can just put 1 week old seedlings in one inch cubes on top of the larger slabs of rockwool? The roots should then just head straight into the larger base?
Or do you cut a little hole in the larger rockwool slab?