How do Terminators fit into watch companies?

I know a watch company is 4 kill teams of ~10 marines each. These can be any of the primaris ones or Deathwatch veterans but how do thr Deathwatch terminators fit into that? Are they a kill team too or some kind of honour guard?

Also, are Deathwatch honour guard a thing? Can I have a squad of squad of bladeguard or sterngaurd acting as "bodyguards" for my watch captain?