Is it possible to make a space where only the legbars can go?
I'm thinking of maybe a fence with openings the Legbars can squeeze through, and the Rhode Island Reds cannot. (I don't know if the RIR are enough bigger than the Legbar for this to work.)
Or maybe a raised feeder, if the Legbars can fly higher than the RIR (maybe sitting the current feeder on top of a table.)
Of course you could separate them into different pens, but I'm guessing you already thought of that and would prefer not to.
Having lots of places to eat can help, as can open feeders, so the Legbars can snatch a few bites when the RIR aren't looking or aren't close. But based on the feeder you show and the one you mentioned, it looks like you are trying to avoid wasted feed or feed eaten by other animals, so having multiple open feeders might be a big step the wrong way for that goal.
(I don't see any obvious solutions, so I just mentioned several things that popped into my head, in case one or another of them gives you any useful ideas.)