I’m pouring a cement pad that’s 11 1/2 ft x 11 1/2. What is the minimum depth I can safely get away with?

Also, I’m pouring this slab inside a dirt-floor barn, that already has a concrete foundation around the sides. Can I pour the new concrete right butt-up to the old concrete foundation? or do I need to use some kind of spacers? If so what kind? wooden? how big? [...]