I use ABG type mix and some home compost. The plants do well in it. a substrate that is mostly clay will pack down too much to grow plants in. It needs to stay light and fluffy for proper aeration, so for this application, just garden soil doesn't work well either. Not sure about mixing sand into the substrate. Some would be ok, but putting snakes on primarily sand seems to be controversial. No experience with Eco Earth. The top surface of the substrate should be DRY. It will be dry with a screen topped drainage layer (absolutely necessary!). New England Herpetoculture has good info on how to properly layer the drainage and top substrate. Serpa is a YouTube channel run by a guy who shows terrarium/viv builds step by step, with emphasis on the construction of the bottom layers.