The new blood will be needed at some point with any breeding program. You just HaVe to be careful IMO opinion in any outcrossing that it is not to far of a outcross. New blood in the same strain from another source I think would be fine,however you will still have to cull. As mentiond before you better be ready for when you start crossing. To much hetrosis popping up for most to hammer back out. I think the no strain crossing is the word of wisdom to the beginners to not get to screwed up. Far out crosses results in a lot of mediocers and differences in the f2' f3, and further generations without some serious breeding and culling
Ok first let me put out this disclaimer to beginner and intermediate breeders: DO NOT ATTEMPT TO CROSS ANYTHING, it is not worth the effort and you will likely cross up some birds and then quite the line/breed SO DON'T DO IT!
Now, Bluetick thanks for you effort in satisfying my need for this to make sense to me. I can sleep with that!