Kevin, glad the answer was helpful. In the example you gave it might be better to continue using the generic resource, incrementing the work to the new estimate when ‘Sally’ comes on board, then re-adjusting the schedule to align with the new finish date.
You’d have to track the appropriate work completed through the time/labor system. The cost system ought to be aligned with the labor charging system (WBS, Work Packages, etc.). If done correctly, using generic resources can work 1) as long as people are charging to the correct charge number and 2) The big caveat is knowing the cost of the engineer levels/grades that goes into the planning. If a senior engineer charges work that was initally planned to be performed by a junior engineer. Thus your SPI could be good, but your CPI would suffer.