The watchman and scooter should be just fine together. They are not in any direct competition for food and territory.
Yes, sand sifters are very good for the sandbed. They sift for food debris trapped in the sand so therefore keep the substrate clean and help keep waste to a minimum.
Here are some pointers for the future tank population....
If you plan on having any type of coral, the CC star is a
It's best to wait at least six months after the tank has been cycled to introduce angels and butterfly fish. Also make sure to feed them special angel and butterfly foods. This food has sponge in it and allows them access to some type of coral protein...which is a must for longevity.
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.