Well, having to be smart with your engineer placement was intended a bit, so you would never actually have enough to fill all those 10 slots (but, to be honest, most of them are not needed - you only need the plant that is the best for you at this moment, for instance).
The engineer cost scaling is 100 (the cost of each next engineer is 100 times higher), but it is divided by the population squared, so the population has a big effect on the cost. It was probably a bad design decision, since it is hard to recover if you lose energy output.