They need to be close to the same size as the other hens if they don't have a mama to protect them.  This is true whether you have a rooster or not, it's a toss up which will be more harsh to them if they're too little, the rooster or the other hens.

If they free range and there is a lot of room for them to escape away from the rest of the flock, they have things a little easier, the new kids will hang together in a little mob and offer each other a bit of protection, just make sure you have two feed areas out of direct sight of each other, otherwise the old hens will keep the new kids away from all food.

If they are in an enclosed run, it's harder because the old hens will corner the new kids and can really hurt them.  It's not called the "pecking order" for nothing.

I'm guessing you'll have to try earlier than would be best because they'll be too big for 8 to fit in a dog cage for long.  Just know that there will be SOME pecking and harassment no matter WHAT you do and it's usually best to let them get it over with.  You won't be able to stop it all.  Unless a mother hen raises them within a flock they won't know the rules for a bit and they really need to deal with it as best they can which is harsh for you to watch.

Really different birds depending on if they're Mediterranean types, or heavy birds or dual purpose birds, and even different breeds within those groups, grow and feather out at different rates, it's hard to tell you in weeks when the best time to try it will be.  When they're fully feathered you just may have to give it a try even if they're a little on the small side and hope that they are big enough and fast enough to catch on quick. 

That's a common method lavenderblue, they'll still bicker in the morning just to work out the pecking order, but it often helps.  They still need to be fully feathered and somewhat close to the size of the older hens though, but it does help to add them at night.