To answer your second question first, Yes. Should be fine.
To answer the first, sand on its own tends to be a bit loose, but as you continue to use the forge, ash from the fire mixes with the sand and makes a mix that stays in place somewhat better.
An old trick is to put a tin can in front of the nozzle of the tuyere, pack a lightly moistened mix of sand and ash around it to form the bowl, take the can out, and build your fire. Once the bowl is filled up with burning fuel, the sand can’t collapse inward.