don't even worry one bit about the nitrates, all you are doing is seeding the sand, and the sand bed will only help with the nitrates, the nitrates in the other tnk wil have almost no effect on what your outcome will be, once the bed starts working they will fall anyhow, our nitrates were over 50 when we started and having no corals that really does not matter alot, but they fell on thier own
you may get some garbage with it, but he amount, i would assume would be minimal, and for using sand like that, you don't need alot to seed a dry bed, and using lr too, you definitely won't need much, the lr will help too
don't clean it at all, just put it on the sand bed, if this is a new tank, jsut put the dry in, and the lr and cycle, then when complete, add the other sand from this other tank just a cup or 2 would be enough, but the more you use, the more life you may get to start it off
HTH