keep them on chick or starter feed til they start laying.
the difference between mash and pellets is just the form of the food - mash tends to be (varies in different parts of the country) the grains/corn/soybeans/etc ground up to a rough mixture, and the pellets are where the basically same stuff is pulverized to flour consistency and made into little pellets.
Some people like the pellets because it is easier to feed. With the mash, which I prefer, you do get the powdery stuff left after the chickens pick out the bigger stuff - think like the powdery stuff at the bottom of a cereal box. You just add something to the powder (water, soured milk/buttermilk/cottage cheese/yougurt etc) and they eat that part up too. I like the mash better because it is less processed, and usually less expensive since it doesn't require a factory with extruders to produce it.
I believe it is also fresher, because, again, it isn't made by a factory somewhere and shipped across the country to be stored in a warehouse and shipped again to a big box store or feed store and stored again, and then sold.
I would watch that bird seed and feed it only as limited treats.
Your chicks are old enough that you don't need to mix the grit in with their feed, just offer it free choice.
What is trill?