Let me just first tell you, this is my first and only experience with chicks, so I have VERY limited knowledge, buuut...
I got my first 3 chicks, 2 golden laced wyandottes & 1 easter egger, and had to wait 1 1/2 weeks for my 2 buff orps to come in.
I have had NO problem with the age difference. If anything, the feisty little buffs pick on the older chicks more!
They woke me up at 3 in the morning last week, so I snuck down to check on them, all 5 were sitting on their perch, (the older were 3 weeks & the younger were 1.5 weeks...SOO cute to see them roost at that age, btw!)
Anyhoo, they were all sitting on the roost, and one of the buff orps was staring at the side of one of the glw's head...she stared for a few seconds...then just POUNCED right on top of her! Scared the poop right out of her, poor baby squawked and ran away, very upset.
Your new ones will be just fine, in my opinion.
Have fun!!