If you are very careful, you should be able to add the new ones. 17 weeks is usually a good age because they should be 3\4 the size of the big ones.I'm running out of space. My 4 weeks old are too big for the brooder and my 17 weeks old are getting so big that they have almost no space.
Can I add the 17 weeks old to the adult coop?
Also, some breeds are better at handling new members to the flock.