Generally I put my chicks out at 6 weeks and at that point start to ween them onto growers though I never have chicks at this time of year. If they are well feathered and you have hardened them off heat you can put them out provided we don't have close to freezing conditions. Certainly by 8 weeks I would expect them to be out. They don't really need grit if you are feeding mash or growers pellets but once you start corn then they should have access to grit. If they are outside they will find enough grit for themselves or if you like you can use normal grit. For the two or three weeks when chick grit might be ideally optimal it's hardly worth the effort of using it when there are reasonable alternatives.
I have never used it and have raised hundreds of chicks.
Generally on the topic of feeding crushed egg shells which some keepers do. If you do this you will need to bake them to kill any pathegens and make crushing easier. However personally I don't think it is a fruitful use of anyone's time and effort. Oyster shell is much more effective as a calcium source, is a waste by-product anyway, and very cheap, a Kg shouldn't cost much more than £1 and for a few chickens will last a very long time. (I've not used more than 4kg in the last 10years for around 24 chx). To get the equivalent amount of calcium you'll need to crush 500 - 600 egg shells so make sure you don't use too much energy baking or crushing because as a green policy it's a none starter. Don't bother and use the time you save to enjoy your chx. However if it makes you feel better don't let my opinion stand in your way.
That should be good for a few posts!!
Regards