If you are using flats of groundcover or even purchasing them in individual plugs or pots, there is sufficient soil already attached to the roots to get them established. One characteristic many groundcovers share is a very shallow and spreading root system - they don't need much soil to keep them happy. I have quite the assortment of various groundcovers growing in several inches of crushed rock under my GC tables at the nursery - little chunks broken or dislodged from the larger containers - and they thrive in virtually no soil at all! I'd fill between the pavers with the sand and plant the plugs of groundcovers as is, with the container soil attached to the roots. As long as drainage between the pavers is good (and no reason it shouldn't be), they get enough sun, you don't walk on it too frequently for a couple of weeks and water as necessary, you should be good to go. No need to fertilize - most GC's require none. This is the method I've used both professionally and at my home. I've got a variety of different groundcovers growing between recycled concrete pavers set in a crushed rock base - no added soil except what was attached as they came from the pots.

loam is good soil.do not use compost. just use good soil. many plants will grow in sand as well.

We have plants between our flagstones and I wish we had put gravel instead. I've tried numerous types of plants/ground covers and nothing so far has worked really well. It is hard to keep watered, weeds proliferate, some plants will grow for a while and then die out. Thyme does well but some of it gets taller than what I want. It is high maintenance and if I had to do it over I wouldn't.

Similar problem, but my stones line a stream in full sun. So xerophiles won't do. Are there mosses that would do well in full sun in Pennsylvania? What I'm really after is something to replace the tall weeds so I can stop weed-whacking along the streambank.