If you're using brushes, scaling them with always affect the look they have ( mostly blurry them) so you have to draw a set with all the several sizes you want for them on your map adn don't scale them. By the way, when looking close up to those mountains, the noise effect you used doesn't look that good and kind of kill the outlining. If you can have access to a scanner, I would suggest to draw them by hand, scan them, import them in GIMP in turn them into brushes. If you don't, try to draw simple outlines without too much effects on them in GIMP ( possibly using some thin light grunge roud brushes. Shwarzkreuz did post some of his own in his DeRust taining maps WIP, maybe it will help you). Hope it helps.