I guess my question is... how hard is it to intigrate.
Sometimes integration is extremely easy, no drama at all. Sometimes it is a disaster, as you have seen. There are a lot of different parts to that and it can be different for different people, even if things seem the same.
One big factor is room. I know you constantly read on here where 4 square feet per chicken in the coop and 10 square feet in the run is utopia. Real life doesn't work that way. The 4 and 10 work petty well for most people if you have a small flock that is already fully integrated and you have no more than one rooster. For many people it is overkill, more than enough room. For some it can be a bit tight but usually pretty good.
But when you try keeping an additional rooster, let a broody hen raise chicks with the flock, or integrate it is often not enough. Trying to integrate chickens with different maturity levels can be worse. There are tricks we use to make it easier and sometimes it isn't too bad, but as you saw it can be really bad.
I feel like the chicks were almost caught up in size.
This is one of the fallacies. Size is not what is important, maturity level is. It isn't unusual for bantams to dominate full sized fowl. It is the spirit that counts, not size. More mature chickens outrank less mature chickens and are often not shy about defending their pecking order rights, especially if the younger invade their personal space.
One way chickens have learned to live together in a flock is that when there is conflict the weaker runs away and stays away. It's not a square feet thing. If they don't have enough room to get away then the stronger doesn't know that they won and keep attacking. If they don't have enough room to avoid it's hard to not invade personal space if the older ones wander in their direction. The older do not understand that they caused it.
I am just worried about how hostile my RiR seem to be... is this normal?
More normal than I wish it were whether you have Silkies, Orpington, or RIR. To me the breed isn't that important, they are all chickens. My juvenile pullets tend to avoid the adult flock until they mature enough to start laying. That seems to be a right of passage into the adult flock. When I have different aged broods out there the different aged juveniles tend to avoid each other. This is not always true. Sometimes the different ages mingle a lot. Often it is much better than I make it sound. But mine have enough room to get away and avoid if they need to.
It may have even been just the rooster as the hens seem more tolerant of the chicks.
At 7 months he may not actually be a mature rooster, possibly still an immature cockerel. There can be a big difference in behaviors between the two. With my flock a mature rooster doesn't threaten the chicks, he is more likely to help take care of them if he doesn't just ignore them. The hens are generally where the threat comes from if there is a threat. But mine are introduced to the rooster when they are very young, he assumes they are his offspring. It is certainly possible he is either too young to have developed those behaviors or they are old enough he assumes they are not his. Immature cockerels and immature pullets just don't always behave like mature roosters and hens. Roosters and hens can be a threat to chicks, especially in tight spaces, but I find juveniles to be more of a threat.
I think the main issue is that my coop is about 2ft off the ground and the chicks probably had a hard time finding their way back after getting into the run
You could certainly be correct. They may not have known how to get back up there or may have been trapped where they couldn't. What often happens if they can't get away is that they hunker down and try to protect their head. The older keeps attacking since the don't understand the others have given up or they don't care.
So what can you do? Housing them where they can see each other is a step in the right direction. You've done that for the last month. It does help but obviously wasn't enough by itself.
Give them as much room as you can, in the coop and outside. That's not always easy. You can often improve the quality of what room you have by adding clutter. From one of your comments I think you understand what I'm talking about.
Your Polish are probably bantams but the Marans are not. You may be able to set up a safe haven area for the Polish (holes small enough they can get through but the RIR cannot) but there may not be enough size difference to benefit the Marans.
Try to set up separate feeding and watering stations so they all can eat without conflict. That may in the coop and in the run but maybe you can do that in the run with clutter to help you.
Try to not force them to share space. As much as you can, leave it up to them when they want to get together.
I'd consider rehoming the younger ones until the others actually grow up, then try some more. I think mature hens and roosters are easier to deal with than juveniles.
