My pleco has never done that to any fish, and he gets on fine with my gar. Like I said they are the best of friends. If the pleco does start sucking on other fish then I will remove him but hes been a joy to keep. they have been known to do this yes, and have been known to affect gars and bichirs but that doesn't mean they all do it. Mine has been fine for a long while now.
Maybe it is because I keep my pleco in a large wide tank with lots of room for him to patrol. I have only had plecos suck onto fish in the past in smaller sized tanks. as it is more of a captive behaviour and not a wild one I guess it would be much less common in a larger tank.
I feed my gar live fish because its wild caught and thats all it knows, so I feed the gar 5 fish per week