Nocturnal yes, though they will have activity throughout the day every now and then. Unless you have a relatively large colony, you really won't notice an extreme amount of food being eaten. Roaches really don't eat THAT much per insect compared to some other insects. Offer carrots or other fruits/veggies periodically and they will nibble on them as they please. You'll probably find with this species that they will drag their food items underneath the soil substrate to eat - this is perfectly normal in burrowing species such as this.

Their abdomens were probably damaged by the way you collected them. You said you were kicking the soil away to find them...instead, perhaps you may try collecting by hand and being very careful. Contrary to common belief, roaches are not indestructible and some can be downright fragile. Handle with care, and extract them from the soil gently.

The substrate (bedding) you have sounds fine with the cocopeat. These guys are burrowers from my understanding (their morphology suggests the same, along with your description about where they were found). They'll likely remain concealed underneath the soil/cocopeat much of the time. While you may be tempted to dig them up occasionally and check on them, your best bet for success will be to leave them alone as much as possible. Cockroaches by nature are quite shy, and excessive handling or disturbing of them can stress them out and prevent them from breeding.

Most cockroach species we use as feeders can be reared very similarly to one another. Feel free to read over this when you have the time, I made this cockroach rearing guide to answer most basic questions on roach husbandry:
http://www.frogforum.net/food-feeder...r-roaches.html