If you are going to feed any that must be broken down by the "chewing" action of the crop, things like grass, grains, solid fruits & vegetables, you need to make sure that grit is introduced into their diet. The grit needs to be extremely small. I'd recommend you buy grit specifically packaged for chicks. I usually wait 3 weeks before feeding anything that falls into that category.
I ground whole oats (literally about 1 tablespoon) in the mini food processor and give once every few days. Makes the poo a bit less runny, at least that's the hope. I do a tiny bit of yogurt too. Mostly, they are fine and happy with the regular chick starter.