You SHOULD be able to bisque un-glazed greenware stacked. Under glazes is a different problem. If you fire below the flux temp of the under-glaze you probably OK. Err on the side of safety. and wipe the bottoms and don't stack them if you are not sure. Maybe stack one or two and try it.
If you are new to this, slow down and learn what will work and not work by testing non-critical pieces.