You should always keep new chicks separated for a period of time before introducing. One reason is for biosecurity, if the new bird had some sort of illness, mites, etc. It would contaminate the rest of your chicks. Think of it as an obsevation period, to ensure good health

Also, I personally wouldn't put chicks of that age difference together. Older chicks naturally will bully the younger ones. It's pecking order and has nothing to do with the breed. All breeds are this way. I keep my chicks seperated if they are more than a couple weeks apart in age. As they get bigger and more even in size then I will put them together, but not until after an introductory period. I will put the younger chicks in a dog kennel or cage inside of the older chicks run, for aproximately one week. This gives them the chance to see each other and get to know each other without being able to hurt the young ones. After a very bad experience ending in the near death of one chick, I learned this the hard way.