We investigate the coupling between a small-core-diameter dispersion compensation fiber (SC-DCF) and a single-mode fiber (SMF) and its applications in fiber lasers. The number of interference modes and the power distributed in the interference modes can be selected by controlling the coupling between the core of the SC-DCF and that of an SMF, which can be used to select the lasing oscillation range. Interference pattern with a narrow free spectral range can be obtained with a shorter length of the SC-DCF based intermodal interferometer comparing to that constructed by the common multimode fiber. With the SC-DCF incorporated in an erbium-doped fiber laser (EDFL) cavity, under three different selected coupling conditions, stable dual/triple wavelength switchable EDFLs in three different wavelength ranges are, respectively, achieved by tuning the polarization state of the laser cavity. We also demonstrate its application in four-wave-mixing-based multiwavelength switchable tunable EDFLs. A stable operation of 34 lasing wavelengths is obtained. Multiwavelength switchable tunable EDFLs in two different wavelength ranges are also achieved by controlling the coupling and polarization state of the laser cavity simultaneously.