People on this forum integrate new chickens all the time in widely different circumstances. Some integrate mature adult chickens, some integrate really young chicks. Often it goes really smoothly but sometimes it's a disaster. There are a lot of different factors that go into that. Things like the personality of the individual chickens involved, flock make-up, how many new and old chickens you have, how much room you have in the coop and outside, how the coop is laid out inside (especially roost area), how the outside space is laid out, how you manage them (when are they locked up or free to merge), and many other things. The more we know about your unique situation the more likely we can suggest things that might help you.
There are always exceptions, but in general if you are integrating a truly mature hen or hens, integration is faster. It also tends to be more violent. Sometimes it is so smooth you don't notice a problem, but often they have to fight their way into the pecking order. In my experience size isn't very important, maturity is. Mine are typically mature enough to do that after they start laying.
If you have enough room and patience, integrating chicks is often less violent but they take longer. Until they reach maturity mine tend to stay far away from the adults day and night. They need enough room to be able to do that. If they are shoehorned into a small space where they can't avoid the adults it can get deadly pretty fast. Depending on what your facilities look like, they may need to sleep separately for months. Some people don't have the facilities or patience to do that.
You mention raising one or two chicks. Chickens are social animals. It's harder to raise one chick by itself than it is two. I usually suggest a minimum of three chicks. If one dies you still have two so they still have a buddy. Usually they don't die but it can happen. It's also usually easier to integrate two or more than a single chick or chicken. Since they are social animals, a single chicken will probably want to be close to the others, which might get it beat up or worse. If they have a buddy they can hang separately from the adults as they slowly work their way into the flock.
My brooder is in the coop, the chicks are raised with the flock. I usually do about 20 at a time, way different to your one or two. Usually by 5 weeks mine are totally merged with the flock. They don't mingle that much with the adults, though some might get closer than others. They tend to stay in a sub-flock, well separated from the adults during the day. They don't sleep on the main roosts with the adults until they mature. Instead they have other places to sleep. When they mature enough to move to the main roosts with the adults it's usually not violent at all.
I don't know anything about your situation. It may be better for you to wait until they are fully mature to try to integrate them. To me that would be the last choice, but your circumstances are different to mine. The more we know what you are dealing with and what you have to work with the more likely we can offer suggestions that suit your unique situation.