Hey now that is coming together quite nicely! I'm glad to here that you've got some forests that you are "almost happy with". Sounds like you've already put quite a bit of work into this, and if you are satisfied with the forests now, great (they look good to me), but I did want to at least let you know of a way around the scaling issue. In a recent map I was working with a canvas size of 5544x2250px @ 300dpi and, like you mentioned, the built in cloud filters just couldn't produce small enough cloud patterns to work with at that scale. The solution I found was to use a pre-made tiled difference clouds pattern (courtesy of Ravells), and then used RobA's wonderful "Scale Pattern" script to scale the pattern to the size I needed to render trees. If I recall, the clouds pattern was a little too light for my purposes, and I think I increased the contrast before using it (but I can't remember the details now).
Anyway, if you are interested, here's a link to Rav's Difference Clouds Patterns (located about midway down the page), and here's a link to RobA's Scale Pattern script.