Basically you have 19 birds, within months you are going to need a coop that will handle that, and while you may be able to cheat on numbers and space this summer by free ranging, you will need to have the number of birds fit your set up come October when the days get short, and the amount of time they roost gets longer and longer. You should be having an 8x10 foot coop for those numbers, excluding the run.
I am with Ridge runner, a tractor just sounds too small for that number of birds.
As to your original question, I would put all of them together at this time. There will be some scuffles, but adding more to less works better than adding less to more. It will spread the pecking out. So what I am saying is adding 10 to the 6 will work better than adding 3 to the 6, those 3 are apt to take a bit of abuse, unless you have had them side by side in a see but don't touch situation.
MRs K