Yes, that is the best time to worm because their laying slows way down. After worming don't eat any eggs for 2 weeks. You should worm in the spring and fall anyway.
Since your girls are 5 mos old the larger shell should be fine. The finer shell was probably just ground finer by the company that packages it. Hope this helps.
They're both a good source of calicum, but I find just giving oyster shell much easier. I give it to them free choice. If you decide to give egg shells, make sure you boil them first. You don't want your chickens getting a taste...