Bro chico,
Think of it this way, it's like attaching a smaller faucet to a large water line, malakas ang daloy ng tubig, di ba? But to my knowledge, Sakura amps have a protection circuit which shuts off audio output to all channels, I have a 1600 model which mutes when a speaker clips, you'd have to turn it off and on again for it to operate again. To prevent getting burned speakers, when the distortion starts to become apparent, that's where the limit should be. Also, the ideal power for an amp to drive a speaker should be 20% higher. That means if a speaker is rated at 100 Watts, it should ideally be driven by a 120 Watt amp, maybe that would give you an idea. I also think it's up to you if you choose to buy different speakers, but you still should base the impedance on what each channel on your amp can drive. The impedance at the main channels are just lower because main channel amps of Sakuras are usually more powerful than the other channels. Those specs only say that you have more options with the main channel speakers and less options with the other channels which are obviously weaker. But to make things easier for you, why not buy similar speakers? All of them having 8 ohm impedances? Again, just look at these as water lines and faucets. Hope I cleared up your confusion a bit.