Switch them all to a non-medicated grower ration. Supplement the laying hens with oyster shell for calcium. The young birds will not eat this.
Personally I would not recommend mixing 6 week olds with mature hens. Ideally you should wait until at least 8 and preferably 10 weeks to allow them physical access. Hens may beat up an 8 or 10 week old, but they can much more easily kill a 6 week old.
I live in Australia and cannot find oyster shell grit ANYWHERE?
I have been giving my chickens shell grit which just looks like shells from the beach that are crushed up and the local rural store said that they are a source of calcium.... is that right?
Yep, that would be soluble grit and it's primary use is for calcium supplementation... If they are not on a commercial feed only diet they should also provide them insoluble grit, this is usually composed of quartz, granite or flint as this provides the sharp edges and hardness to break up natural foods while not giving them an excess of calcium...