They never dig deep so it shouldn't be an issue. I have never had cories in a dirt tank before though, the gravel you have atm should keep soil capped ok with depth of at least 1, 1/2 inches, the only thing with soil is it will leech ammonia (leading to higher nitrate levels). I have tried setting up a small tank with dirt just as an experiment (no fish) and nitrate went quite high and ammonia was 2ppm. After week or two the filter will catch up and ammonia will return to zero. From reading around the best thing to do is to wash the soil in a rubble bucket leave it to soak overnight submerged by around 4inches and remove the surface scum. You can repeat this action to make sure you remove any nasties (excess fertilizers) that may have built up in the soil. Spread the soil out to dry and sieve out all the wood and bits that will break down, you should be left with a dry soil that is light and airy (this is the stage I'm at) apparently this mineralizes the soil and should keep any ammonia down to a level the filter will deal with. You will have to check water quality daily to monitor the effects just to make sure levels of nitrate or ammonia dont go to high. I will be adding a clay laterite to the soil mix as well. In my current planted tank I have just sand and use an all in 1 fertilizer, I dose Iron, Po4 and use easy carbo which helped a lot with growing the plants.
Edit: You may find slow growth if you have zero phosphates as most plant supplements avoid it. when you have an established tank this lack of PO4 can slow plant growth so adding it as an extra daily dosage might improve growth (it did with me)
Soil could give your water a slight tannin (light brown look) as well. not harmful and does add to natural environment not everyone likes it tho.
