While free range chickens will do a good job of eating a wide variety of food, I'd not rely on scratch to be their 'staple' diet. I use scratch as a treat only, not as a staple part of their diet. Scratch feed is like us eating candy all day long - not very nutritionally complete.
Chicks under laying age should be fed a starter chick feed, and granite/chick grit if they are getting any other type of feed in addition to the starter. If they free range, they may get plenty of grit from the environment, but I offer grit as well, as it is cheap and goes a long way.
Laying hens should be fed a layer pellet along with free choice calcium/layer grit like oyster shells, etc. Free rangers may not eat a lot of grit, but I'd err on the side of caution and have it available for them.
Meat chickens should be fed a grower/finisher feed after the chick starter, as per the manufacturer's recommendations.
Anyone else have another take on this one?
PS - I'd take ANY advice gotten in a feed store or pet store with a HUGE grain of salt. They are interested in selling products, not necessarily truly educated about what they are selling. I could go on all day about the just plain old ignorance that you get in pet stores about small pets like guinea pigs...same holds true for feed stores too. Also, what will keep an aminal alive is not necessarily the BEST nutrition for them. I could survive very nicely for a long time on nothing but sweet tea and french fries, but after a time I would not be a very healthy person.
PSS - OK, last edit... please don't consider a 20x20' pen 'free ranging'. While that is a good size for exercising your chickens, depending on how many you have, they will very quickly eat anything of nutritional value. That size of pen will not allow the variety that chickens need to be healthy. I know my girls totally cleaned out a 8x8 pen in about 2 weeks down to the bare ground.
My chicken yard area is 20x80 and I don't consider them free ranging at all. They do have grass and bugs, but they don't have a huge variety of plants, etc, and the longer they graze out there, the less there will be. I'd be surprized if my 8 girls don't have the ground pretty much bare by spring.