peebee is right about load falling. Whatever you do, be mindful that some smart person might just push both buttons at the same time.. interlocking up/down should be considered.
Utility company do cater for additional load. I agree with rbulsara that you should rate the switchgear according to the generator and ensure the right trip level in protection... all in an effort to avoid additional spending and inconvenience if the overall load do increase in the future.
I think that gravel used in substation is meant to maintain a damp area under it so that a low resistance would be present for earthing purposes. it is not my belief that this gravel is intended to absorp oil.