Twb, just throw them all together, I know two pens can be a bummer and they can keep warmer with the numbers.
About the food. The chicks will get used to the adult food. I have the chicks from Brian's incubated eggs that have been with the big chickens now for a couple of weeks. The big chickens get 16% layer pellets. That is what everyone gets. The chicks are still mostly hanging out inside the chicken house, which is very large. When I go out there (several times a day), I give them some grower crumbles. They gobble it up, they prefer it to the layer pellets, so I treat them. If I don't get a chance to get out there, they have to eat the pellets. They have to just suck that up. They are old enough to eat anything, and they have to learn to munch 100% on what the other birds are eating. If I were you, I would just throw them in with the big ones, they will eat whatever they can to fill up, hee, hee. Have a wonderful and awesomely great day, health. Cindi