When we talk about a Klein Gordon field we are describing a prototype, not a real particle. In real life there would be a different field, and a different creation operator, for each type of particle. At the elementary particle level the pion is composite, and so to create one you would need to have creation operators for each of the quarks that make it up. There really aren't too many Klein-Gordon fields to choose from, in fact the only elementary spin zero particle would be the Higgs boson, if even that exists.
But you could speak instead about Dirac particles, in which case there are different creation operators for all the quarks and leptons.