Let us first state that there are no scouts in the deathwatch. Why not? the same reason as that there are no tactical marines in the deathwatch.
Being a devastator, assualt, tactical or scout was what you were before you took the oaths. A deathwatch marine is required to be adaptive and change his style based on the mission. So when the mission requires brute slaughter the marines will take their power armour or even terminator. If it requires stealth they take the scout armour. It all depends on the mission and how they plan to succeed. you can have specialists, but in the end each marine must be able to do what is required.
Secondly I want every one to remind them that becoming a deathwatch marine requires being a hero in his own right. So deathwatch marines have seen many years as a battle brother before they are even considered for the post.