1 is safer. They can be pretty aggressive, particularly around feeding time. If you want one to keep all of his/her gills, I would recommend just one in a 10 gallon.A) how many will be ok in a 10 gallon?
I would go with sand. If you get the really good stuff used for high end freshwater tanks the particle size is really small and tightly packed. Plants can root in it, and it is tightly packed enough that detritus cannot easily accumulate in it.
C) live plants? plastic? silk? - - have heard they can really damage live plants and uproot them....
Either/or work fairly well. They dont like fast water flow, so a small internal filter may be better so you can block the inflow with a rock or something.D) are internal or external "waterfall" filters best?
Large swollen cloaca when sexually mature for males.E) how do you sex them?
Again, probably a bad idea to keep two together at all in a tank that size.F) I do not want to breed, will it be bad if I keep 2 of the same sex together? (are males territorial etc?)