Thanks ahead of time... some of these might seem dumb. 
I am new to chickens and the whole incubation thing. I am, however, familiar with breeding reptile (leopard gecko) eggs, so I'm not going in to this completely blind.
#1 With geckos, cross breeding certain types is frowned upon because you end up damaging the purebred gene pool. Are there ever any chicken breeds that should NOT be mixed with others?
#2 With geckos, there is a limit to breeding back-to-back generations (mother to child, grandchild, etc). 2 to 3 generations is considered okay. What are the general guidelines for chickens? Or is it never okay?
Just want to do things right!

I am new to chickens and the whole incubation thing. I am, however, familiar with breeding reptile (leopard gecko) eggs, so I'm not going in to this completely blind.
#1 With geckos, cross breeding certain types is frowned upon because you end up damaging the purebred gene pool. Are there ever any chicken breeds that should NOT be mixed with others?
#2 With geckos, there is a limit to breeding back-to-back generations (mother to child, grandchild, etc). 2 to 3 generations is considered okay. What are the general guidelines for chickens? Or is it never okay?
Just want to do things right!