Localizability and coupling constants

Let us consider for free elementary systems the postulates: (i) localizability of systems would not favor in a physical sense some inertial frames and that (ii) standard quantum mechanics (or, at least, a skeleton of it) applies for localizability. It is known that, at least for the lower values of spin, (i) and (ii) imply a unique solution of the localization problem for the no-interaction case. Extrapolating the analysis to the case when interactions are present, we offer arguments in favor of theconjecture that if elementary systems under interaction are localizable, then (i) and (ii) imply restrictions on the coupling constants, and probably their single-valuedness.