Frankly, the advice you received from that person is probably the most sensible I've ever heard from anyone working in a feed store. I live within a few miles of six different ones and I've heard the wildest crud you can imagine.
That said, there are many ways to raise chickens. When I brood, yes they have a heat lamp for weeks and rarely go outside until it's in the 70's out there, usually around 4 weeks old. However, when they are with broody mama, they are out and about within three days, as long as the conditions are dry, in weather down into the 50's, but when they get cold, they cheep and mama hunkers down with them. They find their own grit, never get cocci, etc. I prefer that they are fed the age-appropriate feed for optimal growth, i.e., lower calcium levels and higher protein levels than adults. If they eat anything that must be ground, greens or grains, they need grit. If they can't get it themselves, you must provide it. If they are exclusively on starter, they don't need grit; it's fine enough.
I don't feed "treats" to chicks until they are around 10 weeks old, unless you consider chopped hardboiled eggs treats. Some feed nothing but scratch, but I take the condition of my birds over theirs any day. I've seen stunted growth in youngsters from improper feed.
Chicks will survive on many diets, though in my opinion, they may not exactly thrive.
And for the record, I don't think Buff's comment on the title of this thread was inappropriate or "not nice". Some responses here have been, in my opinion, somewhat snotty and rude. Let's stay civil, please, as Wynette has already requested.