@Janroe
Cardinals, guppy or any other fish will likely be a threat sa RCS especially if baby rcs pa murag man gud daphnia. But if you provide lots of hiding places (plants, wood, etc) plus sila pa jud ang una sa tank mas kompyansa sila sa mga fish kay very familiar naman.
In our planted tanks we are actually doing bio-engineered aquatic ecosystem kay ato man butangan isda, tanum, and other invertebrates to mimic the real thing. So in the case of the RCS being eaten by fish, just think of it as "predator-prey" relationship happening in our tanks (better than Nat Geo hehehe). Unless of course your main purpose is RCS propagation then that fish has to transfer. Otherwise just let it be as how nature dictates.