Oyster shells can ABSOLUTELY get rain on them. I actually offer mine in a plastic rain gutter!
Layer feed is cheap, and has extra calcium added to the mix. Whether its enough for your hens or too much or not enough is anyone's guess - the amounts are based on old studies involving commercial layer breeds under commercial management conditions. if you aren't raising commercial layers of quality similar to those used in the old studies, under commercial management conditions, those assumptions don't hold, and the studies' conclusions may be inapplicable to your situation.
@DobieLover says, not for roosters, not for hens either, before onset of lay (much more important than calendar date).
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@NatJ too - typing at the same time I was. Excellent advice. As usual.
For the typical backyard owner, of the typical backyard flock, under typical backyard management conditions, I recommend an "All Flock/Flock Raiser"-type* crumble feed for all of your birds, all of their lives, without regard to age, gender, molting status, or onset of lay. Plus free choice grit, oyster shell, and fresh clean water in seperate dishes, of course.
*For our purposes, "All Flock/Flock Layer-type means an 18-20% Protein feed, about 3.5% Fat +/-, 1.5% Calcium +/-, 3.5% Fiber +/- in that order of importance. We can really get into the weeds and talk about Methionine, Lysine, Phosphorus levels, but start with the big three - Protein, Fat, Calcium. Plenty of "Starter" or "Starter/Grower" feeds also fit this definition. The important things are the mill dates and the guaranteed nutritional label, NOT how Brand X chooses to label the bag.
Yes, you can mix 1:1. 2:1 All FLock to Layer is better - you are trying to reduce the calcium content. Calcium Toxicity is progressive, it builds with time, but it is particularly damaging on young birds, early in their development. Additionally, Layer is based on old studies for ADULT birds - the nutritional needs of hatchlings are much higher as they put themselves together - levels of Methionine and Lysine particularly that Layer feed just doesn't meet.
The example I've given recently draws from human history. A hatchling fed layer may look healthy among its fellows, it may even be healthy - but it will never live up to its potential after recieving a deficient diet early in life. In this respect, its like comparing a human male from North Korea with one from South Korea. Among their own populations, each appears normal and healthy - but put the North Korean male next to his South Korean counterpart and he is (on average) abotu 3" shorter and many pounds lighter - owing almost entirely to the improved diet of South Korea as the stepped from Third World towards First in the last half century or so.
