If you want to work with a pure bred chicken line and improve its egg production here is must read. http://www.albc-usa.org/documents/ALBCchicken_assessment-2.pdf
Few things to think of one must have land to do this for you need many chickens and must hatch many more and grow out to cull...
Here is a link that will help you out.
http://www.ithaca.edu/staff/jhenderson/chooks/chooks.html
You can not buy the parent stock of the hybreds for then the company selling them would not have a monopoly. many are copryright or whatever they do for animals like that.
Yes, Welsumers are pure bred. But they are smaller than the RIR, BR and Autrol. But bigger than Leghorns.
Leghorn will develope fast than all other breeds mentioned. The others are duel purpose breeds and are slower in growth and latter to lay eggs first time. If you mix the two your eggs...