I used to think that "troop" meant a group of soldiers, but somewhere I picked up the idea that it was actually correct for a single soldier as well.  Can't remember where, though.  Let's see what I can find out…  Well, the Online Etymology Dictionary agrees with "troop" for a group only, and doesn't mention a more recent singular sense.  "Trooper", it says here, applied only to cavalry at first, then to mounted policemen; it didn't make the transfer to foot soldiers until the early 1900s.

Various dictionaries agree.  Had they said down near the bottom somewhere that it's also applied to single soldiers, it wouldn't have impressed me; all that means is that the editors of those dictionaries recognize that some people use it that way.  But the fact that that usage isn't mentioned at all is revealing.  I think I'm gonna have to reverse my reversal and agree with you, CyndiLu.