How/When to separate 19 chicks

We received our first chicks about four weeks ago. We ordered 18 pullets and ended up with one extra. They quickly outgrew their brooder and are now outside in a 4x4 coop with an 8x4 run. We ordered so many with the assumption that they would not all survive (had been told and had read not to expect all to make it). We are certainly glad they are all doing great, but now that means an additional coop. We should have the second one finished by the weekend.

My questions are:
1. At what age should we separate into two flocks? Do we wait until they are several months old, or go ahead and do it now before they get too big to all be in one coop?

2. We know that at least one of the 19 is a rooster. Do we wait to separate them until we know if we have another one so that we can have a rooster with each flock?

3. Once we separate them does that mean they cannot be in the same run together? The run we have is temporary (just built it to protect them while they are young). Once they are old enough, we plan on putting them in a larger fenced in area. Will they be ok together in that larger area later if we keep them in two separate smaller runs now?

My main concern here is that 19 is too many for the 4x4 coop, but I still want them to be able to get along once they are separated. I want them to be one big happy family that lives in two different houses but am not sure how/when to accomplish this.

Any advice would be great appreciated. Thanks!