There are a couple of ghoul NCR Rangers at this station, which I found pretty surprising, and what i found even more surprising was there seems to be no information at all about who they are why they are there. Afaik these are the only NCR-aligned ghouls in the entire game, why here? What's their story?

They're not the only ones.

While they are some of the only NCR-Aligned ghouls in the game, they are are not the only ones, the station is primarily set-up to serve as a place of refuge for Private Edwards in the quest Weill All Go Together. (Edwards and others were ghoulified when CL apparently set off a "dirty bomb" in the town's center.)

That would be my reasoning of why Ranger Station Echo has stationed ghoul rangers...