I wouldn't mix them in with your regular girls. Meat birds do better on their own special food - which chickens think is heaven. This wouldn't be so bad if it didn't have a tendency to make your regular layers overweight and if it didn't cost more than average chicken feed (at least it does out here).
Also, meat birds are _extremely_ docile. If you have any huffy hens then your meaty babies may be bullied.
As for numbers, if you want to raise 50 meat birds on your first go then have at, but here are some things to consider:
1. Meat birds are made of meat and poo. I'm convinced a meat bird craps his weight in poo every day. Fully convinced. They eat a _lot_ more feed and all of that feed has to go somewhere.
2. This isn't just chicken poo. This is evil poo, the sort of stuff that can make your eyes water and blow your mind ("That little one week old bird did _not_ make that smell."). It's like having a baby. A real mini-person baby with a very full diaper. Multiply that by 50. This is the sort of stuff scented oils won't cover - it's like putting potporri in a well-used gas station restroom. All it does is smell like herbal poo.
3. This poo doesn't just smell - it's pasty and depending on the feed, it's going to be very runny, and will require regular cleaning.
Meat birds are not chickens. They are birdzillas, somewhat related to chickens, but mutated into doing everything _a lot_. Lots of growth, lots of eating, lots of stinking, lots of poo. We'll raise 300 quail in the house or 100 chicks before we'll keep a dozen Cornish-Rock inside.
I have only somewhat exaggerated these qualities. If you're keeping the birds outside in a brooder with a light and plan on cleaning it out then you should be fine. If you plan on raising them in your kitchen for a few weeks (read: a few days
before moving them outdoors then you may be in trouble.