After a slow introduction yes. I would put the 6 week old and have them penned were the pullets can see them and get near them but not get ahold of them. If you just toss them in one pen all together the older ones will pick on the younger ones. By slowly introducing them and penning them side by side they get use to each other. Of course I'm sure someone will chime in that their birds do just fine when adding new members to the flock. But personally I would rather be safe than sorry.
Also did you brood these 6 week olds or purchase them at this age? I quarantine any new flock members for 30 days before letting them in with the rest of my flock. This helps to alleviate new illness from being introduced to the existing flock.
Like I said before, they need to be introduced slowly no matter what age they are. Also since you bought them at this age they need to be quarantined for approx. 30 days or else risk introducing illnesses to your existing flock.
I put my younger birds in a dog crate in the hen house for a couple of weeks just so the older birds can get use to them and they don't seem to get picked on I mean there are some pecking order that goes on but nothing bad happens this has always worked best for me. Slow intro is deff a must in chicken land.