Hello Connie!
Snails can harbor nematodes when they are wild collected, this is true. Usually a qt period of a month or more is adequate to make sure nothing can be harbored. The snails that I am selling have been qt'd for longer than this already. Nerites will very readily eat all the biofilm algaes first (specifically diatoms is very palatable as its soft). With your temperature, you would do best with the clithon corona snails or sun snails. I keep them with my discus and they are doing very well. Your hardness is on the low side, but I think with how tough the nerites are they would do quite well for a long time before you noticed any erosion. Most other snails cannot tolerate that temp range. I have had excellent luck with the c. coronas though in my discus tank and think they would be a good choice. Please feel free tomessage me if you need more information.