A pair would probably be ok, but things might be different when they breed. It's hard to tell. Also, the rcs would be at risk of being snacks...how many guppies and otos?
RCS will most def be a snack on the list with KRIBS guppies and otos should be ok if they are big enough, otos are all over the place and the guppies should stay near the top and the kribs will stay at the bottom mostly and with RCS they love to stay near the bottom as well and will be teriyaki shrimp..