So I introdiced my 21 week olds to my 6 weeks olds and they got along really well surprisingly, I was wondering if could possibly start putting them in the same run together during the day it's 24 by 12 feet
The best time to integrate is around 16 weeks of age, when integrating chicks with older chickens, I have found. If you do it before then, they may get cornered and at at that age, they may not stand up for themselves. The older ones can gang up on them and try to kill them. Sadly, everything may look OK and you turn your back and come back to find one pecked and bloodied. I have gotten away with it before 16 weeks, but it is a risk. At 6 weeks of age they don't stand a chance unless they are raised by a broody and she will protect them, from my experience.
It can be done despite the size difference as long as you provide safe areas for the smaller chicks to retreat to when chased or bullied. I usually build a temporary "panic room" in one corner of the run with entrances ( more than just one because a bully can always block one) into it that are 5" x 7", too small for the big chickens to get through. A big consideration when integrating smaller ones in with bigger ones is the food and water situation. The pecking order being the unforgiving code that it is, requires the older ones to attempt to exclude the smallers from eating and drinking so it may be hard for them to get enough to eat unless you place their food and water in this safe area. Other ways to provide safety is to add stumps or old furniture to the run so the smaller ones can run behind or jump up to safety. Adding extra outdoor perches also helps.