My question is, would getting an older roo and putting him with the older flock be wise or a recipe for disaster?
I personally like raising my chicks, male and female, with a flock that has adult hens and a mature rooster in it. I think it mellows out the maturing process some, though it can still get pretty wild. To me the big advantage is that the mature rooster goes a long way in reducing how much the cockerels bother the mature hens. Mine don't interfere that much with the behaviors of the cockerels with the pullets their age.
I've raised chicks by themselves, not adults at all. When they matured the hens and rooster knew what to do by instinct. I've raised chicks with mature hens and no mature rooster. When the boys start feeling the hormones there is occasionally some conflict between the head hen and a cockerel but usually not a lot. All the hens don't beat the crap out of a cockerel when he start feeling frisky. If they don't want to mate they usually run away. That's where the dominant hen is more likely to interfere.
Having raised them those three ways, I think the advantage in having mature chickens in the flock is over-rated. There can be some benefits like the mature hens will show the pullets where to lay eggs (and sometime they pay attention) but I don't find any real differences in behaviors of the adults regardless of which way they are raised.
I don't know how much room you have. I find the more room they have at any age and in any mix of age and sex the fewer behavioral problems you have and the less severe they are likely to be.
Personally I would not bring in an older rooster now or later for bio-security reasons. I prefer to raise my own from eggs I hatch or with chicks from a hatchery.
If you bring in an older rooster eventually the cockerels will mature to the point there will probably be some conflict with a mature rooster. It may be a fight to the death, it may not. You can have the same issues with more than one cockerel.
Unless you really want that mature rooster as your flock master I just don't see any advantages in bringing in an adult rooster, whether you bring in older hens with him or not. I also don't see any advantage to waiting a few years. I just don't see any real difference in the behaviors. You can have issues either way, you can have a real smooth process either way.