I'm planning to do the mixed breed dual-purpose flock too. I'll be doing Buff Orpington rooster on Buff Orpington, Barred Rock, and Rhode Island Red hens. I've found that the hybrids grow out better and are hardier than the purebreds. So I'll put some eggs under a couple broody hens and keep or sell the pullets and eat the roosters.
One thing to consider though is that you'll want to have a separate place to keep the roosters if you plan to keep them past five months old. They'll start being a pain in the neck at around four months old, and by five months old they need to be separated from the hens and pullets. In my first year of keeping chickens I had some hens and pullets literally raped to death. At best, the hens and pullets are stressed out, leading to slowed or delayed laying. Just thought I'd mention this.