On the discussion of Scouts in the Deathwatch, the scout character featured in Dawn of War 2, Cyrus, is noted to have served in the Deathwatch. Not only that, but it's stated that Cyrus served in the orginzation for nearly two centuries before deciding to return to the Blood Ravens so that he could bring back all of the knowledge that he learned to his chapter. He was also noted to have been a veteran of a some campaign against the Tyranids during his time in the Deathwatch, though I don't remember if the opperation was successful or not.
Much Like Tellion and Naaman, Cyrus is a full grown space marine who chooses to be a scout and wear the armor, opting to train the neophytes in the ways of battle in a (mostly) standard codex chapter. As has been remarked by Davian Thule, "Cyrus is no fool. There is a reason he chooses to remain a Scout Sergeant. In a few centuries there will not be a Blood Raven left who hasn't been molded by this thinking."
So, no, I don't think the idea of having Scouts seconded to the Deathwatch is all that weird.