I've let my chooks go without grit for a while when I know they've got access to pebbles, though sharp grit's necessary in general. It help to chop up any long bits of grass or whatever they eat that potentially would bind them up and kill them. Sharp (normal) grit chops that stuff up in both the crop and the stomach.
I've given chooks small-bird's grit temporarily while they don't have larger grit, it should be fine. Chicks should ideally be getting small grit as soon as they are keen, but most of them seem to obtain that from their own eggshell to begin with, at least they do when hatched naturally. On a random note, if you observe any of your poultry eating a long piece of grass rather than doing their best to remove it, that's a silly bird that's got the greatest chance out of all your birds of being the one that gets crop or gizzard bound in future. I've noticed that with mine; those who persist with swallowing over-long pieces rather than trying to be rid of them seem to come down sooner or later, almost as a rule.
