I have been raising the same cross you are for about 5 years now. You can feed them the starter the whole time and butcher/sale them at about 10 weeks. But this can be pretty expensive.
At 4 weeks when I start letting my chicks go outside i start mixing the feed 50/50 chick starter and scratch grain. I use the Purina chick starter and their scratch grain. They do a really good job of making sure the corn is cracked into small enough pieces for the little chicks to digest. There are some local places that sale their own blend cheaper, but it seems to be less processed at the mill.
I hope this helps. If ya have any questions just give me a shout.
Cheers and best wishes
Nick