How many 2.5 year olds? And if it is more than one, you are basically doubling your flock. Do you have room. If you are tight on room, it can make it more tricky.
Putting them in a see no touch really can help. But mostly it depends on your set up, how big it is. Basically, your birds are younger, which means that they are lower socially, however, it is their home turf, which raises their status. Being as they are all nearly full grown I would put them together fairly quickly. There might be a bit of bluster, but probably not too much.
If you are getting two or less more birds, I would let my own birds out of the run/coop into the yard, put the new birds inside the run/coop for the day. Letting my own birds back in near dark. I would feed along the fence for both sets of birds. However, I do have a lot of space, a lot of hideouts, a lot of roosts, in my run and multiple feed stations.
But measure your coop, and tell how many birds, cause that can change things.
mrs K