It probably won't hurt. But be sure they get the protein they need to finish their bone growth. Layer feed is usually much lower in protein than growing chickens need. Giving them all a can of mackerel once a week ought to cover it.
I would say you are ok. I use flock raiser and use oyster shell on the side for a year now. but do what you want. Now what I do any time I change feed or add meds or electrolytes to water is make a mash. They like a mash and get a taste for the change of the new dry food or water without turning their nose up to it.