Without seeing the soil it's hard to say if it needs something added, but generally I'd say no. Sand is added to heavy soils for drainage, clay (powdered or just clay rich soil) is added to sandy soil to give it more stability to hold tunnels/chambers better. I don't think it's needed, my desert scorpions are in 8" of straight sand and have burrows and chambers right to the bottom and I've never had a collapse yet.
For most of my set ups I have a false bottom, a layer of gravel covered in fly mesh under the substrate, with some I don't add the mesh but I use rounded pebbles to avoid injuries. I wouldn't bother in a small set up for ants but I would with an aquarium. The important thing about a false bottom is the tube that has one end in the gravel and one above the surface, this allows you to add water to the bottom of the substrate without soaking the top, the gravel is just to allow water to flow across the bottom before being absorbed by the soil. The gravel also avoids having a saturated stagnant layer of soil on the bottom of the enclosure, it allows for aerobic bacteria growth rather than anaerobic bacteria growth which makes soil smell bad. Personally I'm still using a straw to add water to the bottom of the substrate gravel or not as the natural order is that soil becomes more moist the deeper an animal burrows so an animal can adjust it's own moisture level by the depth it burrows. For ants I'm simply poking filter material into the straw to keep the ants out.
You're right that sterilisation is going out of fashion, the theory is that soil contains a balance of bacteria and other organisms and sterilisation leaves the soil open to colonisation by the first bacteria or organism that comes along. It's still a hotly debated subject in most forums, it's your choice really.
The way to get sand or any substrate to hold burrows and chambers is to compact it as it's added, add some, firm it down as much as possible then add more, compress that and so on. Then I get it very wet, spraying the stray substrate from walls, decor etc and getting the substrate much wetter than you want for the animals involved. When it dries to the desired moisture level it will be very solid and hold burrows/chambers very well, even sand will set rock hard, you just need to leave time for it to dry.
Wooo! gr8 amount of info there, I already made the setup sadly tho, its a few inches tall of dirt, didn't add anything to it, seems like strong dirt too so hopefullu good, added somerocks between half the soil, not sure if it'll do much, I might poke a straw through the bottom tho, I am definitely going to take some of ur suggestions on my other soil setups. thanks!
