You might follow the link in my signature to get my thoughts on how much room chickens need. I don’t give you hard and fast numbers, just things to think about. You’ll see some magic numbers on here of 4 square feet in the coop with 10 square feet in the run per chicken. This is meant for someone keeping a few hens all the same age in an urban back yard. You’ll also reads a lot about how many people are out there regularly managing poop. Chickens poop a lot.
Your instincts are right about those prefab coops. How well they work out depends on your overall system, not just the coop in isolation, but they tend to require a lot of work, you don’t have enough room to manage any problems that might pop up, and if you leave them locked in there any length of time (for predator protection or just because you want to sleep in on a Saturday) they can have serious behavioral problems if you come anywhere close to believing their capacity claims. Not that they are guaranteed to have problems but the likelihood goes up the more you crowd them. You can probably build something a lot bigger and more suited to chickens for less money.
Chickens are living animals so it is hard to say exactly what they will do. If they are raised together they tend to get along pretty well though you can still have individuals that are just brutes. Plenty of people have those walking toilet brushes or chickens wearing blinders in with their flock and don’t have any real issues. Occasionally they get picked on. Occasionally other chickens get picked on. Occasionally other chickens get picked on by them.
I find maturity more important than size when mixing chickens. More mature chickens are higher in the pecking order than less mature chicks. They call it the pecking order for a reason. If a less mature chicken invades the private space of a more mature chicken, the higher ranked chicken can enforce its social status in the flock by pecking. That doesn’t mean it will every time, just that it might. What you find with chickens of different maturity levels is that they tend to form separate sub-flocks. While they may sometimes mix with impunity, the younger tend to avoid the older until they all mature enough to sort out the pecking order and get along.
Space is critical to this. One way chickens have learned to live together in a flock is that when there is conflict the weaker runs away from the stronger. The weaker usually learn really quickly to just avoid the stronger to start with. If they don’t have enough room to run away and get away or to avoid then it can become deadly.
I don’t know how much age difference you are talking about when getting those Silkies. Some of us don’t have a lot of problems mixing younger chicks in with the flock. Especially if they are crowded you sometimes need to wait until they are all practically grown, and even then you can have serious problems. It’s really hard to predict what living animals will do. The individual chicken’s personality has a lot to do with it too.
I suggest you get that second coop ready so you have a backup in case it doesn’t work out. But don’t let the possibility of something not going well stop you from trying. Worst comes to worst, you have to house them separately. It probably won’t come to that.
Good luck!