With smaller rocks like the ones I alluded to in the original garden in this thread, I don't think there would be such a problem. Assuming, of course, that the gardener is not periodically cultivating the soil or disturbing it in some way. With large rocks, after the first 1-2 years of settling, very little movement occurs. Two caveats to this statement though: (1) if the soil is naturally loose like beach sand, all bets are off (is this likely what you have in Florida?), (2) it is common in gardens that the rocks don't actually sink, rather the plants and soil build up around them, giving an illusion that the rocks are sinking. Don't forget that the amendments you add, i.e. compost and mulch, don't decompose to nothing, even though it seems like it.
