What is the main goal of teaching fluency to students beyond 6th grade?Should you teach fluency in isolation to students above 4th grade and beyond...

There are two possible ways to view this question, so it depends how you mean.  I would assume since you mention isolation that you are referring to teaching English as a Second Language, so let me address that possibility first. This is a matter of opinion, but I believe teaching in tandem with comprehension is a better method, as I think we tend to overemphasize pure fluency, and in the years that takes, we sacrifice a lot of learned basic skills, especially for those in younger grades.  For example, I would think learning basic math would be more important to a 4th grader than fluent sentence structure.  Math is universal in language.

If you are referring to teaching foreign languages to English speakers, then  I believe the Rosetta Stone isolation method is the most effective for achieving rapid fluency and language mastery at any age, but very much so with young learners that acquire language more easily than adults.