Unfortunately they are going to get pecked - it is part of natural chicken socializing and whenever you introduce new birds they have to re-establish the pecking order. This comes with a little violence.
I would put your tweens in a large wire dog crate or some other fence/cage so that all of the birds can see each other and be "together" without really being together. This will give them a chance to get a little used to each other. I would do this for a few days and then put everyone together. They will likely be mean but you have to get them time to work it out.
Watch them and intervene if you see blood. Take out the injured bird and let her heal fully before putting her back. Also watch the make sure everyone is getting to eat. If bully around food becomes a problem them perhaps add a temporary second feeder or waterer.
I've also read that once you do let them mingle for the first time it should be while they're free ranging, if at all possible. So introduce them when they're out roaming rather than penned in. I haven't done it myself, but I've read that advice several times so I figured I'd pass it on to you. HTH!