Regarding the center channel for two channel playback, it does require an additional speaker and another amplifier, which is mono. Center is R+L and you can add two back channels for ambience with two more speakers and a mono amplifier, the two back channels are R-L. That makes a total of three more speakers and a stereo amplifier. The front speaker has to match the L & R speakers, while the ones in the back can be smaller bookshelf as long as they don't sound too different from the fronts. The amplifier should be the same as your main amp.
While L+R gives you mono without noise, L-R gives you ambience plus noise so back in the analog days, that is when LP playback and FM were your only options, adding noise to the rear channels was really annoying. Now with digital sound, it might be well worth revisiting this concept.
Why is this not done? Well, it is. Almost all home theater receivers have all channel stereo sound and the internal DSP does the work for you. For us purists, well, I don't know why. I know at least one of us on this board have done it as I was at his house when we experimented with the concept. The big advantage of this approach is the analog processing is all passive - no active devices are required other than the amplifiers.