You look confused. In my opinion, with only 3 years experience in homing my own flocks isn't so much the issue of breed. As it is the method of raising the animal. Although some breeds are larger than others, which would put them at an immediate advantage in any spat, it mainly boils down to temperment. Rir are known for their wild temperment as a breed, but that does not mean they are all this way. I currently home 12 rir layers who are very gentle birds in general. They occasionally peck at me while I care for them, however these instances seem to be from affection. Not to get too far off topic, it would come down to pecking order. Smaller birds are naturally at a disadvantage in smaller enclosures.
So if you have an abundance of room to house your mixed flock, and the proper time to care for them the smaller birds can become part of the family.
Keep a close eye while introducing your birds as well as over time as pecking order changes as time goes by. No need to end up with harmed animals.
As far as adding a roo to your flock, it would mainly depend on two factors.
Size and temperment, which tend to work hand and hand when breeding any animal.
A smaller roo breed may not be dominate to a larger hen, however the larger rhode island red roo may be too rough to breed with smaller types of chickens. Think of it more along the lines of a German Shepherd breeding with a shitzu.
Therefore it comes down to compatibility and would differ from bird to bird.