First, Scotch moss is not a moss at all but a low growing mossy-looking groundcover with white flowers (mosses don't flower). And it is not well suited to much shade. It likes a rich, moisture retentive soil in full to part sun. I doubt your Scotch moss will do very well for you in that much shade.

Second, consider using your rocks to make a dry stream bed. Depending on where you site the bed and how long it is, you may need to get a few more rocks, specially some with a bit more size to them. Dry stream beds can be useful in directing water away from certain areas during heavy rains (sometimes placed where drainage/runoff normally occurs) but even if it never sees any runoff, a dry stream bed is a great way to show off low growing rock plants tucked in here and there along its sides. It can be a very attractive addition to a garden. Just try to make it look as natural as possible. The link above illustrates the process and the end result very nicely.

Thanks gardengal!

Nice advice from gardengal. To be honest, I wouldn't bother rinsing them, let alone trying to polish them; let the elements do that for you. I vote for the natural look, too. Trying to 'polish' these babies is going to take materials on an industrial scale. I used to tumble-polish stones, and it's great fun - but you could only do about 1lb at a time... I warn you though; once you start a stone hobby, it's hard to stop. You'll see certain stones and think 'Hmmm, that would look nice....' and before you know it, your pockets/bags will be bulging! :D

Around the birdbath question of planting? Have you considered Helexine? The plant does die off in the winter, and appears to rot off, but it recovers wonderfully in the spring!

More info.

I have dug out hundreds and hundreds of pounds of boulders (and pavers) from my garden bed, all dumped there by a previous owner who was trying to something with them I'm sure. I don't know what the exact vision was, but over the years the area got covered with soil and mulch and neglected until we came along. I pulled them out and used them for edging, with the larger ones placed in various spots in the beds for decorative touches. I never bothered cleaning them off other than maybe scraping off the larger bits of clinging mud and soil. Let the rain take care of all that for you and they will be clean in no time.

Sorry that you suddenly found yourself doing much more labor than you anticipated!