Considering adding a sinker intermediate board to my quiver. Looking at the 4'11 primarily for wing foiling but I would also like to use it to help perfect my pop up and on smaller days for prone. Just curious if anyone is actually using the smaller sup/wing boards for prone as Armstrong suggests you can. I already have a SKT 4'5 that I use for kite and prone. Still terrible at my pop up though. Sold my bigger 5'6 prone admunsun to get the SKT and I think I just jumped too soon.

Oh and I intend to keep my fanatic 5'4 94l board for light wind winging, and I have figured out the stinkbug start but riding the SKT with the wing won't happen very often here in Florida. Feels the the 60l board could be a ton of fun for winging in 15+ mph winds.

I cannot see why not it would be okay for a prone. I'm not 100 percent certain but I can't see much difference in overall design between the wing -sup and the wing -surf boards. I have a 4'5 FG and have seen plenty of wing-sup boards in the flesh and I'd say they have pretty much the same design.
If anything the width is the crucial point - too wide and pain to paddle and yawing becomes an issue when riding the wave. 4'11 is what 23.5' inch? That will be fine width wise. They are a super stable platform imo when popping up.

I've proned my 60L at 63kgs and didn't have any problems (other than lack of duck dives).