The only pro I see in using fleece as a substrate is that it is washable. I would use reptile carpet or just newspaper before fleece. Make sure your using a thermostat since fleece moves around easily and the snake can rest between the glass and the substrate easier and get burned. I like to use a substrate that the snake cannot access the bare bottom of a enclosure and rest directly on the heat source just incase. Also if humidity is a problem the bedding will do no good. Good luck though!