with the pullets only a few weeks apart how would that work? Does it take as long?
You never know until you try. It could be as easy as putting them together or it could take time and effort. As always, the more room they have the more likely it is to be easy.
Even when they are adults it is not always hard. A lot depends on the individual personality of the chickens involved. Some are so laid back toward other chickens that integration is easy. Others can be a real terror. You cannot predict how they will interact with a new chicken based on how they interact with the current flock. The meekest mildest hen can become a brute. It's a try it and see.
The other really important part is how much room they have. I think that is the true source of most of the horror stories about integration you read about on here though sometimes it is the personality of the individual chickens. It is not about square feet per chicken. It is about how well can a chicken that needs to run away can get away or just avoid the bully to start with. That means they need some distance and places to hide under, behind, or above (what we call clutter). What I typically see is that when they are first together the younger or weaker try to approach the stronger and get pecked. They very quickly learn to keep their distance, to avoid the stronger. They form a sub-flock, peacefully coexisting as long as they keep their distance. It helps a lot if they have widely separated feeders and waterers so they can eat and drink without challenging the stronger. It's not always like this. Sometimes they mingle a lot, even chicks with adult hens. But it usually is sub-flocks.
Don't forget about nighttime. It is possible your two groups will be able to sleep together, it's even possible some of the older will mother the younger. But mine tend to not sleep together, keeping their distance at night as well as during the day.
What do your facilities look like? How big in feet or meters are your coop(s) and run(s)? How many chicks do you have in each group? Photos of your facilities could really help We might spot something specific to comment on. Without knowing anything about your facilities I like Rosemarythyme's generic suggestions. It's how I'd go about it.
