Soft shelled eggs??

Hi there, I'm fairly new to BYC but I must say it is addictive!! I've read several posts from different people about soft shelled eggs and in general it seems that this can be common in newly laying pullets. I have 7 pullets between 19-21 weeks old which are kept in an enclosed area - coop is 10 x 12 and the run is 12 x 14 along with four roosters which I am trying to reduce to two including a bantam. Anyhow - we have gotten as many as 4 eggs in a day (1 being so soft it basically had no shell). I feed the chickens some yogurt and cheese depending what is in the house and I've taken to crushing up any egg shells we have around and feed them that (scatter the pieces over the run like scratch - not their favorite but it disappears) They also get layer feed. This morning when I went to let the chickens out there were two paper-thin shelled eggs on the floor of the coop along with 2 barely there/no shell eggs on the poop bar. Yesturday I found 2 hard shelled eggs on the coop floor so the eggs I found this am must be from different chickens - I'm guessing 6 out of 7 must be laying?? I've asked the 2 feed store suppliers about crushed oyster shells and they both looked at me like I was nuts-I'm guessing others they supply to must not use oyster shells?? Anyway, I'm wondering what else can I use to strengthen the egg shells - I was okay with one egg being soft here and there but 4 in one night seems like a lot even for new hens? This is my first time with laying hens so any assistance you can offer would be great - also - how much of other calcium suppliments would I need to offer the hens to see any improement if this is the problem? And - is it possible the stress of having three roosters fighting over them could also impact they new egg processing systems?