below) entering $\left\langle V_{epi}(\mathbf{q})\right\rangle _{q}$. Furthermore, since $\left\langle V_{epi}(\mathbf{q})\right\rangle _{q}$ is independent of the the oxygen ($O$) mass, then $T_{c0}$ is *mass-independent* in leading order with respect to $T_{c0}/\Omega$. This means, that in $1ML$ $FeSe/SrTiO_{3}$ (and $1ML$ $FeSe/TiO_{2}$) one expects very small $O$-isotope effect ($\alpha_{O}\ll1/2$). Note, in [@Johnston1-2] large $\alpha_{O}=1/2$ is found; ($\mathbf{2}$) the *self-energy* $\Sigma(\mathbf{k},\omega)$ at $T=0$ is calculated analytically which gives: ($i$) the positions and spectral weights of the replica and quasiparticle bands at $T=0$ - all this quantities are *mass-dependent*; ($ii$) the slope of the quasiparticle self-energy for $\omega\ll\Omega$ ($\Sigma(\omega)\approx-\lambda_{m}\omega$) is *mass-dependent*, since $\sim\lambda_{m}\sim M_{O}^{1/2}$; ($\mathbf{3}$) in the $EPI-FSP$ model (without other interactions) the critical temperature for $s$-wave and $d$-wave pairing is degenerate, i.e. $T_{c0}^{(s)}=T_{c0}^{(d)}$. The presence of non-magnetic impurities (with the parameter $\Gamma=\pi n_{i}N(E_{F})u^{2}$) lifts this degeneracy. It is shown, that even the $s$-wave pairing (in the $EPI-FSP$ pairing mechanism) is sensitive to non-magnetic impurities, which are *pair-weakening* for it, i.e. $T_{c0}^{(s)}$ is decreased for large $\Gamma$, but never vanishes. It is also shown that for $d$-wave pairing $T_{c0}^{(d)}$ strongly depends on impurities, which are *pair-breaking*. The curiosity is that in the presence of non-magnetic impurities $T_{c0}^{(d)}$ in the $EPI-FSP$ pairing mechanism is more robust than 