That should work, although you will have to reattach it to the slate occaisionally as it grows. I use hairnets to fasten my Riccia to rocks. I find it a lot easier than tying it down. There are numerous other methods of affixing Riccia to objects or holding it on the bottom of your tank. Some weight it down with metal bars, others sandwich it between plastic mesh. As long as you can find a way to keep it held down and give it light and CO2, it will probably grow for you. It's a very robust plant
I like to use fishing line but I have also used hair nets and they work just fine! if what you want is a "carpet" there is only one way to make it last a little longer. Before you even start aquascaping, plan ahead what you want. Then start getting your plants ready if you are to use Riccia spp. as a foreground try to get the rocks of your choice to be about the same size.
Place the Riccia spp. on top of the rocks and tie it down they way you like the best (keep them moist all the time)
Place the rocks about 3 inches from each other until the whole foreground is finish.
If you what to do something different in all the empty spaces between the Riccia spp. rocks plant Golssostigma elatinoides or Eleocharis acicularis for a more natural effect.
Hope this helps!