Delete based on how much space is left on disk?

I have several drives which fill up quickly because we use them to store video. We manually delete regularly to clear space for new files.

Hazel can easily delete files older than a certain age, but can you combine that with looking at the space available on the disk and only delete if, say, there is less than 5 GB available?

I assume you are referring to the trash options? If so, then having Hazel clean up based on space left is on the feature list for future consideration. Right now, the best you can do is to limit the size of the trash itself, unfortunately.