I would not use any kind of loose substrate, especially sand, due to possible impaction if ingested. Mine are on paper towels which I know are not glamorous but they provide a smooth surface for them to eat crickets and such. Slate is a smooth surface as is reptile carpet. However reptile carpet can retain germs etc if not thoroughly scrubbed.
Paper towels are easy to keep the environment clean……
Yes, that ratio should be fine. If you want a cheaper option, organic topsoil can work as a substitute for reptisoil. As for the play sand I have seen plenty of people use it without any issues so I wouldn’t worry about using it.