Yes, you can import your QT movie as clip and edit it. How much of a quality hit could range from negligible to noticeable – depending on the amount of compression that was done to the original sequence.
If you only have the final...you will have to convert that to an editing codec, like ProRes. But if it's 8 years old...it might have been SD...DV at that. So converting to DV/NTSC might work. Or as ProRes NTSC. But know that compressing it twice means it won't look as good as the original at all. And yes, depending on HOW compressed the final is...it could be bad, or decent. If really compressed, like a one min spot that is 56MB H.264...yeah, that's pretty darn compressed.