Most of the time, Rule #2 is good advice. [Rule #1, if you haven't guessed is: create a schedule network so that the critical path is revealed; Rule #1 is ignored by those that work only with milestones]
Some of the time, Rule #2 has unintended consequences, like making the critical path longer! How does this happen?
The problem arises when we move from the abstract of 'headcount' to the real world of 'Mary' and 'John'. Now, the parts are not interchangeable. Now we must consider not the generic staffing profile for a task but actual capabilities of real people.
Now we must consider the intersection of the staffing plan with the schedule plan. When two plans intersect, the results are not always as we want them. Intersection means overlap, and overlap means that the planning elements must be moved about so that each overlap is harmonious.
Take a look at the following figure for Rule #2:
You can probably see that if not for the specific assignments of Mary and John, the critical path could be as short as 50days, not 65 as shown.
Let's violate Rule #2 and invent Rule #3: Reorganize the network logic
Staffing does not actually start on what was the critical path, but the overall schedule is shorter nonetheless. A new critical path is born which incorporates the sequencing constraints of the original path and the staffing constraints brought about by Mary and John.
Obviously, on any network of non-trivial scale you can not really do this by hand; it requires a schedule optimizer and the optimizer has to be configured to try, at least, violating Rule #2.
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