Is the old/small coop portable? If so I'd put it next to the new one, so the chickens can see each other and interact through the wire.
If the old coop is not portable, your best option is to rig some kind of tempoary chicken wire enclosure next to the new coop, or even sectioning off a part of the new coop. Your goal is to let the chickens have maximum interaction with each other while still being protected by wire. After 2 weeks or so of this, then let your old ones go in with the others, loose, and keep an eye on them.
If your hens are still larger than the new ones, they're size will balance out how few there are, and you should have minimum fights. But its a tricky thing to do, because one group will almost always gang up on the other. Whether that is full grown hens picking on the youth, or the crowds of young ones picking on the few elders.
Good luck!
PS: another thought. At the point that you let them all loose in the same enclosure, take 1/2 dozen of the young'ns and switch them to the small coop for a week. That will disrupt the pecking order of the main flock, and give you older hens more of a chance to find their own places withing the larger flock.
