Can I put my cake directly on the cake board covered in florist foil?
If you KNOW for sure that your foil is food safe---the original packaging or website from which you purchased says so-- then it's fine. If you don't know for sure, then NO. The colors in florist foil have lead in them and can leach into the cake.
Truth is, food safe florist foil is a bit hard to find and is pricey when you do find it. The stuff at the cake deco shop I go to isn't food safe stuff.
I always put a cake board the same size as the tier under it and then put that on the foil covered board. I stick it down with royal icing or glue dots.