With regards running rear speakers....
If you want sound from all directions then go for it mate, you need to remember it's your system for your enjoyment, not anyone else's....
If running rear speakers, it's always wiser to use an external amplifier as this allows greater scope with regards volume control & crossover points, this added flexibility will enable you to integrate the rears much better than is possible with most head units....
With regards inverted subs/speakers....
As previously stated, this is done for a few reasons, (shallow enclosures, looks etc etc), but in some cases, or rather with some subs, you can pick up audible motor noise or chuffing from the pole vent....
With regards Amplifier & Speaker matching....
An Amplifier capable of delivering 100w rms per channel will only deliver that power whilst playing a 0dB tone at the correct power supply voltage, (usually 14.4v)....
Music can't be recorded at anywhere near 0dB, so your Amplifier will never achieve full rated power whilst playing music, (a fair assumption would be approx 50% power on music)....
The Amplifier will also struggle voltage wise, very rare to achieve 14.4v inside an amp....
You can then add impedance rise due to variables such as heat, speaker cable length & guage, enclosure type & size.....Getting too complicated now, will just say your proposed matching will be fine.....