On this forum, the convention is for each person to start a new thread with their question, even if it is a similar question. Sometimes a similar question has a different answer in different cases, and sometimes it just gets confusing to keep straight which details go with which question.
In...
If the new chicks seem weak or slow, I agree about keeping them separate until they are doing better. A piece of hardware cloth to divide the brooder would be one way to do that.
But if the newly-arrived chicks seem lively enough, I would put them together right away. I wouldn't worry about...