TSC is pretty notorious for mixups when it comes to their chicks, so it's possible what they told you about the age of these chicks wasn't 100% accurate. Either way, their age isn't super important, I just thought I'd point out that they look a bit younger than estimated. 
Just take things slowly and keep an eye on things when they're together, and you should be fine putting them into one flock, especially if they have plenty of space in their coop and yard.

Just take things slowly and keep an eye on things when they're together, and you should be fine putting them into one flock, especially if they have plenty of space in their coop and yard.
