For components other than reactors, it depends on what you mean by 'maximum efficiency.' Ships will use all components at rated capacity, under the assumption that power is available to do so, and will not reduce output or 'turn off' components to reduce power consumption even if the ship does not really need to make full use of the rated capacity (aside from ships not sprinting except in combat situations); when insufficient power is provided, the total output is reduced by a simple scaling factor of [actual power available]/[rated power requirement], which to my knowledge is applied to the sum of the component capacities rather than componentwise (so if you're hoping that your power-hungry Proton Thrusters get shut off before your more efficient Quantum Thrusters, dream on).
For reactors which use different fuel types, the answer is essentially "no." The game will only ensure that a ship has fuel, it will not ensure that the fuel is apportioned in a manner which makes sense based upon its reactors. You can design a ship which has enough Fusion Reactors to power its hyperdrive and cover static needs while enough Quantum Reactors are provided to cover its needs in combat, but the game will not ensure that the efficient Fusion Reactors are the only reactors powering the ship during hyperjumps, nor will the game ensure that the ship has sufficient Caslon and Hydrogen to operate both sets of reactors at full power in combat. You may well run into a situation with such a ship where its entire fuel supply consists of Hydrogen and as a result the ship's combat performance will be crippled because it was designed under the assumption that the Quantum Reactors would bear the greater part of the power demands of combat. The only way to avoid having a ship crippled through not carrying enough of each type of fuel is to have the ability to fully power the ship off of either type of fuel, which essentially means that you have twice the reactor capacity you require for the design.
For reactors where you have a high-efficiency low-output reactor and a low-efficiency high-output reactor which uses the same type of fuel, I would expect that the game simply uses the average fuel efficiency when underutilizing the reactors rather than taking the more complex route of shutting off less efficient reactors first, but it's not something I've tested. Ignoring the NovaCore Reactor, there's not really any set of reactors in the base game which all use the same fuel type that has a high-efficiency low-output and low-efficiency high-output pairing.
< Message edited by Aeson -- 3/7/2016 2:06:53 AM >