I have been feeding my CX with laying mash and just recently, in the past week or so, added sunflower seeds, oats and a little cracked corn.
I have also not fed free choice but just once in the evening along with my layer flock. They have also free ranged since 2 wks. old.
From the pics I've seen on this forum, my birds are not too far off the normal size for their age...if not spot on. I've had no health problems whatsoever. The calcium in the layer ration won't harm them for no longer than these birds are going to be alive.
That is a great deal for laying mash and I would take it if I were you and I would have to agree with the previous post.... alot of the high protein feed is going to wind up on the floor in a hot, nasty squirt.
I would think that buying more expensive feeds, using the same feeds as commercial growers and trying to produce meat just as quickly is no different than birds produced commercially. Might as well save your time and effort and the money spent on feed and birds~not to mention the smell and the labors of processing~ and buy them from the local store.