To answer your first question: if you were to play a mode from begining to end, you would start with the root of the mode. But if you're using a particular mode to solo, you don't need to start with it.

The difference between natural minor and other minor scales (the most common are harmonic minor and melodic ascending minor) are the diferences in intervals. Here are the intervals for the following scales.

Natural Minor: M2,m2,M2,M2,m2,M2,M2
Harmonic Minor: M2,m2,M2,M2,m2,A2,m2
Ascending Minor: M2,m2,M2,M2,M2,M2,m2

In case your unfamiliar with the abreviations, M2 is a whole step, m2 is a half step and the a2 in harmonic minor is an augmented second (a step and a half).