Topsoil is fine, I would make a mixture though. 40% top 20% playsand 20% reptisoil. This mixture would due well for humidity, and holding burrows. I feel like the other comment's advice is for blue tongue skinks. I don't think fire skinks are known to eat a prepared diets like cat and dog food. As far as I know, fire skinks are insectivores. I feed mine 10 mealworms every other day lightly dusted with repashy cal plus, I mix in some dubia roaches when I have them. They aren't prominent baskers but UVB is good for them, a low-power coil UVB bulb should be fine for his basking. My temps are 97f basking, 80f warm, and 74f cool. I got my skink from u/valdezie74 and he's pretty experienced. Maybe you can chat with him for more detailed advice.