Does anyone know much about these guys? For instance
1. Will MTS eat dead MTS in the substrate?
2. Can they handle small gravel substrate mixed with lava rock?
3. Do tons of them actually harm anything?
I don't know of this being an issue to the biological filter (the fact that they die in the substrate), since they all do it and I've never heard of anyone having an issue with this....
They live for a couple of years and as long as you don't overfeed your fish, then you won't have an overfed MTS population. They will eat algae and any other food left behind from the fish. If you have cories or shrimp there won't be much of that though.
I know I have some because every once in awhile I'll see one out of the blue on a plant or something but other than that I never see them. They are very good for sand and I would suspect very small gravel as well, keeping it aerated.