How to create a generic sum?

it really depends, mainly on your clock speed. normal VHDL:

a <= b*c;

will work perfectly well at infering multipliers at slower speeds (like up to 250MHz). But if you need to push the multiplier to the limits, you need to be using the megawizard multiplier as inference wont take into account all optimisations.

Also remember you should only do 1 mult per clock.