In your case, since it's only a 20g, I wouldn't hesitate to empty the tank out and start from scratch. If you're prepared and have everything ready, I can't see why it couldn't be done within a couple hours. What I would do:
1. siphon out as much water as you need to house the livestock temporary
2. transfer your livestock over
3. siphon the rest out into another container to be reuse
4. remove the sand
5. scrub down your tank (optional)
6. rebuild your reef
I have a BB fish only with about 70lb of liverock sitting on 1/4" glass and it seem do be doing just fine. The bottom is completely covered in coraline and it blends in very well the the rocks. IF you can carefully place your rocks in a secure manner I wouldn't bother with any other media on the bottom.
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