http://secure.campaigner.com/Campaigner/Public/t.show?7v0ju--3wknh-zhnr095&_v=2TD, these are purebred rocks. So won't be breeding these with hatchery stock. I'd need to find a different purebred strain if other genetics were needed.
I'm really looking for a starting point. I don't know how many birds I should get from this source. There sold as a straight run so say I buy 30 chicks. I know I'd need or have to cull some of the cocks.
I don't know what is practical genetics wise to know how many to start off with.
I will not pretend to be an expert. But the breeder that article is linked to has really good breeding articles and you should sign up for his newsletters. And he has a website, has written books, fb page and the newsletters are free.
I would also think it would be great if you can find a breeder, group or poultry club of the same kind of bird that would be willing to mentor you. There is probably a Plymouth Rock Club that you can be a part of.
I personally don't think 30 is an outrageous ## or birds, if you can afford it. I would cut back to the best cockerels and hens. Keeping in mind that you want to keep enough to have back ups in case something happens to you best 2. And keeping in mind what ## of hens you end up keeping so you can have enough roosters to cover the hens. Only keep the best. I am sure there are specific issues that crop up with that particular color pattern and breed that an expert can advise you on.