Dubias and mealworms will almost instantly start to burrow in the soil. Red runner roaches do not burrow.
Slings are scavenger feeders. I’ll smash the head of a tiny mealworm, and add it to the enclosure. For really small slings, I’ve actually used the rear leg of large crickets as food.
There are lots of tiny little lessons that you will learn about raising tarantulas successfully. Watch a lot of videos. Tom Moran, tarantula collective, Dave’s Little Beasties all are super. Arachnaboards is also a good forum. But we are always on here as well as others. And we can help with any questions.