Space Marine Scouts have been a vital part of the forces of the Astartes since the days of Rogue Trader , yet the Scouts and Chaplains were not included as classes in the Deatchwatch RPG. Now, mind you, with the right choice of talents, skills, and equipment you can make a passable Scout using the Deathwatch Tactical Marine speciality... but this seems like the kitbash that it is. Why not have it as a full, proper class?
Usually among Chapters that adhere closely to the dictates of the Codex Astartes all the scout squads of a Space Marines chapter are just "Space Marines in Training," organized in the 10th Company. They are learning the ropes and still having the last of their superhuman organs installed before being promoted to full Space Marine status and deployment in one of the Space Marine Squads. (Until the most recent Codex they were promoted to Tactical Squads and then into one of the specialit squads: Assault or Devastator; Now, thet do it backwards.) Obviously, it would not be appropriate for the Deathwatch to draft these raw initiates... That's okay though!
The "trainees" in Codex Astartes adherent Scout Squads are always led by a Veteran Sergeant - a full marine with many years, decades, or centuries of combat under his power armored belt, who has been assigned to the 10th Company to train the newbies. This sort of character can be modelled rather well using the Deathwatch Tactical Marine class. But I still think we could use a more Scout-y Scout. Especially when you consider some of the non-Codex Astartes adherent Chapters' mant variations on Scouts: Raven Guard, Space Wolves, etcetera...