I would think that 500 GPH or even 300 GPH would be too much,
If you wnated to use an AC 500, Id go with a sponge (Great big sponge) on the intake to kind of diffuse the water flow, as well as provide massive mechanicle and biological filtration. I would also go with two sponges, or a bunch of bio-media in the media well. Depending on how many feeders you are trying to keep at a time, you may not have enough tank. I keep my breeding stock of guppies in a 20 or 29g and raise the fry in a 10 g, I only keep a couple dozen adults at any one time, but as said before, All of my fish in the past have been more than willing to eat prepared food so the guppies are just a treat. Check your PM's, I just a had a lenghtly thought that might help as well.