Hi Neurowiz.
First off, upgrade to 2.4.x as there are a lot of big improvements.
Gimp doesn't have specialized layer types/layer effects like photoshop, and no way to dynamically scale the fill
One of the 2.4 features is that the current clipboard contents are available as both a brush and a pattern. So open your 600x600 pattern file (don't bother to save it as a pattern) and select all, copy (ctrl-a, ctrl-c in windows, I don't know the linux shortcut). Now it shows up in the pattern palette as the first entry and you can drag it onto an image (or use the paintbucket tool).
If you want a smaller scaled pattern, just rescale the 600x600 image and select all, copy again, then paintbucket the layer. (Then undo the rescale on the 600x600 image). Not too convenient, I'll admit, but workable. (I'm thinking a plugin might be necessary here.... "Scale pattern to clipboard"...)
Then just use a layer mask to restrict where the fill shows up (like in photoshop). You can change the fill by re-flood-filling (just make sure you select the layer icon, not the mask icon!)
-Rob A>