I have an 8x4, with 15 head in it, 5 young birds are going on 4 months, and I am developing crowding issues. I would think you need to keep them separate, all day and all night. And a bigger coop than you are proposing, but not a lot bigger. 6 x 8 should get you through. You could have two doors, and use chicken wire to split the inside into two.
I agree with the production reds, they are quick to lay, fast growers, and consistently so. As far as handling, one could do it at night, when they have gone to roost. One could get black sex links and red sex links, and have a quick visual separation of the flocks. Sometimes birds of one color tend to hang together but not always.
When people talk about free range, they mean very different things, from the basic back yard, to the south 20 acres. Those different places have different predators. If you are in town, with a traditional back yard, neighbor dogs are apt to be your biggest threat, especially if your birds get out. Sometimes hawks.
Out in a rural situation - coons, hawks, eagles, coyotes in my country are my biggest threats and they can be serious, but regardless of your situation, a predator attack can wipe you out, or seriously interfere with the experiments results.
In a science experiment, one want to keep all the controls consistent, so one can test the variable. In a biological experiment, the individual chickens are going to have a great deal of variability within each bird. What I would hypothesize is that you are going to have different birds make different gains, and lay at a different age...in both groups. While a flock of 14 seems like a large flock to you, it is not really big enough to make conclusive conclusion. It is not enough data. If you had at least 100 birds in each group, then your results might be more conclusive.
If this is a school project, it might easily fit the requirements of the class, and the results will be interesting, but probably not as clear cut as you are hoping.
Mrs K
I think that if you build a coop, and divide it as I suggested, and then build small feeding areas outside of each door. Mark your birds, let them out in the morning into this small area - feed only enough to consume in 30 minutes, and then let them out together. They MIGHT return to their side in the late afternoon, to be fed again. And the few that get it wrong, could be caught and put back to "their" side.
Otherwise, it is going to be nearly impossible to keep apart