Size has nothing to do with it. People integrate bantams with full sized fowl all of the time. Maturity, personality, and how much room you have are the important things. And you are dealing with pullets, whether they are laying or not is also huge in relationships.
You get no guarantees with living animals. You never know for sure how they will behave. We can tell you how we'd approach something and what results we'd expect, but you never know when you'll get a whacko chicken where the "rules" don't apply. Still, most of us are usually successful, there are guidelines that usually work.
For your first integration you'll have 3 hens that are probably laying and already integrated and 2 pullets that are not laying and probably won't be for a month or two. The two pullets may or may not know each other that well. I don't know how big your areas are (coop or run) or how they are set up. I'd want to house the pullets where they can see each other but not get to each other for a week or more. Then, when you can be around to observe, let them mingle. See how it goes. What I typically see is that they form two flocks, the pullets avoid the older ones until they start to lay, then they become one flock. During the day they stay separated, at night they do not sleep on the same roosts. Widely separated food and water stations help so the pullets can eat and drink without being bullied. Hopefully you have enough room to do this.
Then, a month later, you will probably have 3 hens that have been laying a while, 2 pullets that may be laying but probably not and 3 new POL that are probably not laying. I'd go through the same procedure. House them for a week across wire from the rest of the flock for a week or more and then let them out when you can observe. It's possible the two non-laying group will merge. Maybe you have three separate flocks for a while. Or something else could happen. My only goal for integrating is that no one gets hurt. Nothing else, just that no one gets hurt. All that mingling and becoming one flock will happen later.
Some people do have success by just placing new chickens on the roost at night. Some people have disasters. Remember, no guarantees one way or the other. Imagine this for a bit. Tomorrow morning you wake up and there are two total strangers asleep in your spare bedroom. How might that go? Now imagine that they are neighbors that you know well enough that they knew where you keep your emergency house key outside and just did not want to disturb you. Which do you think might go better?
That's not really a fair comparison because it can work, especially if you have a decent sized coop. When I'm integrating younger birds into my flock, when I go down in the morning the young birds are on the roosts while the older ones are on the coop floor. The younger have found a way to avoid the older. So, if you put strangers in the coop after dark they may be able to separate the next morning if they need to. Or it could be a bloodbath. Before I lock younger birds in the coop overnight, I want them to show that they can coexist during the day with the older, even if they stay far apart. And I'm down there at the crack of dawn as they are waking up to see how it is going. Once I'm comfortable they will be peaceful I can sleep in but if it is not peaceful I can open it up and let them out.
Good luck!
Unfortunately, they will be arriving at different times.
