Agreed. For future reference, the feeding instructions on the bag should give you the age at which it's safe to feed any particular feed. I believe most layer feeds recommend no earlier than 15-16 weeks of age because of the reasons Azygous gave. 🙂
If you want them on pellets, I'd skip the layer crumble and just transition them right from the chick crumble to pellets. I do this similarly to how you describe in the first post, mixing the crumble and a little bit of the pellets together, gradually adding more and more pellets to the mix...