Oh Axerax mentioned a good point: different styles.
However, when it comes to spriting, there are very few things that really MUST match for sprites to go together: Proportions, Shading and color.
This template solves the first two: It gives a straight proportion style, and the shading is very simple.
So if all of our spriters keep up with the same rules for shading (a darker tone for outlines, 3 for regular surfaces and 5 for shiny surfaces) and keep with similar color palettes, the rest is just concept and will eventually match as long as the concept given does.
Meticlan: Nice words.
Venetia: Regarding being overzealous, I know exactly how it is, but setting a resolution that you will finish and searching for a practical way to do things (estabilishing a quality goal which is enough to make you proud even if it is not perfect) is what keeps me up with my project
(aside from TLE, where I follow directions and there is a team, which makes going straight easer).