Hi Muhammad,
As far as I understand, the only difference between MMF and SMF is that there are number of modes in MMF and the power can be coupled from one to another. so while in SMF has only one mode. So to make it simple imagine MMF has only one mode (Under certain condition, like excitation, bending, manufacturing condition….etc) so simply it is equivalent to SMF.
Now the second point when should you consider the nonlinearity in the system, the answer is as simple as that when the nonlinear length is less than the fiber length. To meet this condition, you can increase the pump power, or increase the nonlinear coefficient (gamma); by increasing n2, or decreasing the fiber area, or decreasing the operating wavelength of the operating mode.
Hope this clarify the point Muhammad
thanks
Mohamed Abouseif