Sony Receiver / Yamaha Speakers? help!

I want to purchase new speakers for a 7.1 receiver that I purchased last July. The receiver is a Sony STR-DE995. The power rating (reference) is 100 W/ch @ 8 ohms 20Hz-20kHz,THD 0.09% and 110 W/ch @ 8 ohms 1 kHz, THD 0.7%. I am interested in using (2 ) Yamaha NS-777's or NS-555's for my mains and (4) Ns-333's as the surrounds. I plan to keep my center channel speaker - a Yamaha NS-AC143. The NS-AC143 is rated @ 8 ohms, 10 - 160 watts (89 dB/W/M). The 777's (89dB/2.83v/1m) & 555's (88dB/2.83v/1m) are rated @ 6 ohms, 100 watts nominal power and the 333's are rated @ 6 ohms, 60 watts nominal (87dB/2.83v/1m).

Isn't ohms the amount of resistance? I've seen it said that it's harder to drive speakers with lower ohms, but wouldn't more resistance (higher ohms) be harder to drive?

I've seen different calculations:

Amplifier Output = Amplifier Watts x (Amplifier Rated at Ohms / Speaker Chain Ohms)

Amplifier Output = 100 watts x (8 ohms / 6 ohms)

Amplifier Output = 100 watts x 1.33333

Amplifier Output = 133 watts

Does this mean my receiver will put out 133 watts?

Will this system work OK? Will using speakers rated at 6 ohms hurt my receiver? Will there be any problems using an 8 ohm center speaker, while, my other speakers will be 6 ohm? Please help, explain, or make suggestions.