When thinking about the flow, I've always read that you don't consider the return from a skimmer. Unless you can direct that flow, then it's not really adding much to the current in the tank.
You can't really compare the flow put out by a Koralia (or other propeller pump) to the flow from say cannisters or MaxiJets. The flow from a Koralia is so dispersed compared to other sources it's really not the same. For example, I replaced a puny Maxijet 600 with a Koralia 2. By the specs, the Koralia is like 4x the output of the Maxijet. But you wouldn't know it by putting your hand in the tank.
Guess my answer is "... it depends" because there are too many variables to say "yeah... you need X number of Koralia". How you position them, if you bank the flow off any rock or glass, etc all play into the equation.