Well, it would be a good thing if the chicks were very close in size before putting them together. Because the big ones will just beat the crap outta the little ones....they may even kill them. It's just the nature of chickens........
There are alot of things you can do to make the transition easier for them, like at first, let them see eachother but have a little fencing between them (or other barrier) so they can see but not touch eachother. Then let them loose after they're used to looking at eachother.
If you've got room, give the newbies a place where they can go to hide, eat and drink, where the bigger ones can't get to them.....no matter what, there will probably be some skirmishes....until they work out their pecking order......