Is Spectral Coherence (MSC) dependent to the initial amplitude of the comparing signals?
From the definition of the (magnitude-squared) coherence

$$C_{xy}(f)=\frac{|G_{xy}(f)|^2}{G_{xx}(f)G_{yy}(f)}$$

with the cross-spectral density $G_{xy}(f)$, and the power spectra $G_{xx}(f)$ and $G_{yy}(f)$, respectively, it is clear that scaling of $x(t)$ or $y(t)$ does not change the value of $C_{xy}(f)$, because the scaling constants appear in the numerator as well as in the denominator and cancel out.

The coherence is a normalized quantity (and hence scaling-invariant), and it always satisfies $0\le C_{xy}(f)\le 1$.