It states in the info for the Hydroelectric Plant that you can open your buildings immediately without the need to collect the personnel required to run that building but only if you acquire the plant with 22 country bucks. I played the game from scratch and held on to my country bucks until I reached level 22 and subsequently purchased the plant. It made no difference to when I could open my buildings either existing or built post purchase.

If anyone has any info on how to get this feature to work then please feel free to update this.