I have already performed the fit using ROOT::MATH::Fitter class in python, and it works.
I think the minimizer class is something I may use. I would need specifically to draw contours of the chi2 for arbitrary values of the chi2 (Contours from minos for arbitrary sigma level), and you mentioned that I could use ROOT::Math::MinimizerOptions::SetDefaultErrorDef( value). Now it is not clear to me if I should use then the minimizer class to draw the contours or the TMinuit class. (Do I understand correctly that the classes T<Something> are considered low level?)