I wasn't able to find any details on any KRK passive monitors that would have RCA inputs, so here is some general information about setting up passive monitors:
Before using the speaker outputs of a powered amp such as the Px-2208 you want to check the specifications of the speakers and make sure the the impedance matches the output of your amp. For example, the output on the Fender mixer requires a 4 ohm speaker array. Mismatching your power amp to the wrong impedance can damage your amp.
One of the advantages of powered speakers is that the amp in the speaker is specifically matched to the speaker. If you do decide to get powered monitors then you could either go directly from the 2i2 into the monitors, or use your mixer and send through the line outputs from the board.
Usually passive speakers have wire terminals of some sort for the inputs. I have mostly seen screw down, spring clip, banana plugs, and 1/4inch TS jack inputs. In general there will be a positive and a negative input. In some cases such as passive stage monitors there is a "through" jack that lets you chain the monitors together. I think it is unusual to see RCA jacks for passive speaker input, although I think I remember some bookshelf stereo systems using them. If the KRKs have them they may have had a power amp that was paired with them.
Either way, I wouldn't daisy chain your speakers since that would only give you a mono output. Your mixer has two amp outputs, so you should use the outputs to set up left and right sides so you can use stereo pan mixing. You can put the left and right outputs of the 2i2 into separate channels on your mixer and pan them to either side.