With regards to additives in their food, one thing you will want to increase is the amount of calcium that they get. That leads to nice strong egg shells and prevents any potential issues down the line with eggs potentially breaking apart inside the hen.

I'd recommend two sources of calcium. The first one which might be hard to get hold of is crushed oyster shells. Second one is to take used egg shells, let them dry out for a few days and then crush them down into a powder/crumb. You can either add the powder to their food, or give them access to the crumb size bits on demand. I think some also add to water.

One thing i will say, and I know this is hard to do in some cases, but if you have a chick that is weak and cannot hatch from the egg themselves, it is often better to let them pass, especially if you have plans to breed more from your chicks when they grow. A chick that is weak is genetically likely to pass something on to their offspring, making a larger percentage of those weak as well. If you save the chick and then use it as a meat bird later on, then fair enough, but I would recommend against breeding with the weaker ones (mark them up with a dot on their head, not red in colour, until they are old enough to be ringed)

Quite often, the ones that cannot hatch will have lost humidity in the shell from the mother moving off with the other chicks. You can usually tell this is the case if you have to peel a sticky membrane off the chick's down feathers. It's beneficial to keep the general humidity and moisture levels up if you are doing this. I used to use a small spray bottle and mist them with water if i was trying to save them. The drier they get, the harder it is to get them out.

Pumpkin and pumpkin seeds are a great supplement. They are a natural chicken wormer and will keep their digestive tracts free of parasites which a lot will pick up on a natural diet

Maybe i should write a post on poultry husbandry. I think i could go on for hours about them.