yes. i personally will only by plecos over 2inch, otherwise they can be a bit fragile. so thats sounds ok. they grow to about 7" give or take a bit, and should get to 4" in about 8 months or so. from then on its a bit slower. from 6-7" is the slowest
no plecos are fine in a sand substrate, i've just always used gravel from a maintanence point of veiw. just dont buy if it has a sunken stomach. any other things would be obvious that it is unhealthy. do you know if it Hemiancistrus subviridis or Baryancistrus demantoides? i dont know why these two havent had a seperate L number