It depends on their temperament, possibly, but don't forget to keep them isolated for a while to find out if the new ones have any diseases, worms or lice, mites, etc etc. 30 days is usually considered the minimum! Not doing this is pretty risky. you're risking one group or the other or if they each have something, you could have a real mess on your hands otherwise.
When you are ready to introduce them, do so very cautiously, cause older established hens can be merciless in picking on younger birds, to the point of serious injury, lost eyes, serious bloody wounds etc.
The good thing is that at 20 weeks, they're probably the same general size, and they'll have each other to support. They'll probably face off in two groups and act tough ... if you can divide a little space for them to be out of reach, but get used to each other that might help. also, some say to slip the noobs in at night when they're roosting, but I've never had much luck with that.