I added grit at about 6 weeks. Mixing small amounts of fine grit in the starter feed worked great. The only bad things I've heard about grit is when a bird decided to eat all grit. That's why I mix it rather than having a pan of grit. Moving them to layer feed is a preference thing just like feeding the medicated foods. If the size of the feed pellets are appropriate for the size of the chick, feed it.  Right now, I am feeding my adult birds starter feed for a little more protein because of the very cold temps. I add crushed oyster instead of grit at the moment for calcium. There is nothing to worry about when it comes to feed as long as they are eating. A little more protein for younger birds is good. Layer feed is a good maintenance feed. I haven't fed medicated foods before. If you don't have a large number of chicks, and they are happy/healthy, maybe try another starter feed. 
No worries, they will be just fine! Read about growing fodder. It's really easy, and great for the flock!