i would not recommend it. barr/nile perch (assuming the two act similarly, i have only kept barr longer-term) definitely do not act just like datnoids. mine and others who i know who have had them definitely were aggressive fish. barrs eat a lot, move quickly, and grow really fast...you will have difficulty maintaining the growth equilibrium with your gars between the two types of fishes, and at the same time the gars will beat on the the barr because of the silvery fish issue.
it really puts your gars at risk of spooking nearly 24/7. i eventually had to clear my tank of gars just to keep the barr, and then eventually sold it off because i wanted to return that tank to keeping gars. they are eating machines...definitely impressive fish, but not meant in combination with gars IMO and i would advise strongly against it. unless you're fine with high probabilities of broken backs and shredded fish--
--solomon