Mixing 'meat birds' with layers or dual purpose?

I've never raised chickens but I do know that Cornish X's are not very active. I'd be afraid they wouldn't be able to eat as well with more active birds and may be picked on more often. 

There's no reason you couldn't mix varieties, but I personally wouldn't mix meat birds with laying birds for the simple reason that chickens are very social creatures and develop bonds with their fellows.  When the time comes to butcher, the sudden absence of so many of their flockmates can stress the remaining layers and even put them off laying for a while, especially if they've become bonded with one or more of the birds that ended up being slaughtered.

It would also be helpful if you're feeding them special feed for laying or for meat birds, since it's a real challenge to keep one group out of another group's feed if they're in the same run.  If you're not concerned with that and aren't worried about a possible lull in egg production following the slaughter, I would say go ahead and mix them.  If you still have concerns, do some research on the subject and find out what all the pros and cons would be.  I am admittedly a novice chicken owner and still have a lot to learn myself.

I have always had layers, nothing too fancy. But this year I have gotten a hankering to try a bit more.

I have been looking very hard at the Delaware breed. It was developed as a dual purpose breed that is a good egg layer, while also gaining a nice size for meat, albeit, at a slower rate. But I am thinking instead of butchering all at once, do a few here, a few there.

Last year, when I pulled the excess roos, it did change things in the flock dynamic, but did not affect egg laying.

MrsK
 

It is not a good idea to mix Cornish Xs with regular chickens for a good number of reasons. 

First and foremost. They do not move well, and sit still all day, which makes them prime targets for pecking and cannibalism (they are delicious after all).  I'd be surprised if the laying flock held off 5 hours before starting to pick at them.

Additionally, they should be fed on a higher protien feed than the rest of the chickens (assuming they are an older established flock). And depending on how you are raising them, they should have limited access to the feed, whereas the rest of the flock should have 24 hour access to the feed.

You can see where keeping them together would quickly get obnoxious, and more work than it's worth.  They usually only hang around for 8 weeks anyway, just build a temporary pen, and save yourself the headache.

I raised a dozen cornish cross with my layers last year--I think I had mature hens and 4 month pullets, plus roos. I had no problems. I've seen here about pecking and cannibalism but had no problems, even with the cornish that got carried over too long and were 12 weeks. I did have free range, so that may have made a difference. I fed everyone the same food--an all-in-one and left it available most of the time. I feed enough the feeder is empty at night, then refill in the am.

Depends on your set up, I do think space is the key. And I had non-aggressive/gamey hens, that may have had something to do with it. My hens were hatchery dual purpose--rocks, orps, sex links, etc.