Shrimp should be fine with Dwarf Puffers, I have one in my tank and the puffer shows no intrest in it. Also oto's seem to be fine with them as well.
For a black gravel you can use the Hagen Black River Gravel from AE:
[URL="http/www.aquaessentials.co.uk/index.php?...roducts_id=1208"]http/www.aquaessentials.co.uk/index.php?...roducts_id=1208[/URL]
Its not quite as fine as sand but its black (i think AE used to have it in thier sand section so i assume its must be quite fine but not as fine as sand).