My RCS currently reside in a fry grow out tank, and they nibble on everything. They clean up the plants, a bit of the algae as well as left over food. They seem to particularly love eating left over thawed pea that I feed my fry occasionally. I've seen four of them together for about 2 hours picking off the same fragment of pea, which ends up disappearing thanks to them Try to make sure any food that goes in the tank has no or very very little copper in it (should be in the ingrediants list). Although there is debate as to whether copper content in fish food harms them (rather than just copper present in the water), it's still better to be safe than sorry IMO
I think that you should still supplement their diet, because they deserve their own feeding instead of scavenging off the meager leftovers of the other inhabitants.
I would suggest feeding Hikari Crab Cuisine or Hikari Algae Wafers, or, even better, if you can grow an algae farm outside and feed then the algae with all those bacteria and protists swimming among it...the guppy fry might enjoy that too.