Audio Systems: how to size an amp for 4 pairs of speakers, db amplifier, speaker selector with volume control

Question
Hi,

I want to connect 4 extra pairs of speakers (in other rooms) to a 7.1 system.
Research by this amateur suggests that (since I want to be able to run the other speakers without running the 7.1 speakers) I need another amplifier plus a speaker selector to drive the 4 pairs of other speakers (is that correct?).

I have    
Yamaha HTR-5063BL 7.1 Channel 630 Watt AV Receiver
(http://www.amazon.com/gp/product/B003CP0KGY/ref=wms_ohs_product_T2)
connected to Onkyo SKS-HT540 7.1 Channel Home Theater Speaker System
(http://www.amazon.com/gp/product/B000GU78Z4/ref=wms_ohs_product_T2) in a small room,
I am assuming that I can connect the Audio Out RCA jacks on the receiver to some amp to drive the additional 4 pairs of speakers.

I was leaning toward something like this for volume control on each of the four speaker pairs:
Niles SSVC4 4-Zone Speaker Selector With Volume Control, 100 watts per channel RMS; 200 watts peak $225 www.nilesaudio.com
(http://www.amazon.com/Niles-4-Zone-Speaker-Selector-Control/dp/tech-data/B00022L...), do you have any better suggestion?

I have no idea how to decide how large an amp I need. The four pairs of speakers that I want to connect are 8" Polk Audio RC60i (http://www.amazon.com/gp/product/B00006BMQT/ref=wms_ohs_product_T2 8 ohms, peak power is 100 watts, in the ceiling of rooms in my house that are under 160 square feet) using 12 ga wire.
Occasionally I will want to crank it up but mostly it will be used at low to moderate listening levels.
I tried the amplifier calculation tool and suggested values at http://www.crownaudio.com/amp_htm/amp_info/how_much_power.htm using distance = 5M, Desired SPL = 100 dB, Sensitivity = 90 dB, Amplifier headroom = 10 dB and got 2500 watts; persumably that calculation is per speaker meaning I need a 20KW amp for 8 speakers, which sounds nuts (and it doesn't ask how much power the speakers can handle). Do you take 100W times 8 speakers equals 800 watts for the amp?

I assume that the volume control on the receiver will not control the volume on the four pairs of speakers so I would like a volume control on the amp.

I would greatly appreciate advice:
- how to size the amp and perhaps even a suggestion for a specific one (under $500, preferably under $300, was considering this 150 W amp, but perhaps I need more power - http://www.amazon.com/Dayton-Audio-APA150-Power-Amplifier/dp/B000VKXLBO/ref=cm_c...)
- whether you have a better idea for the speaker selector

Answer
My recommendation, since you are mixing systems - 7.1 to 4 or 4.1 or even 5.1 - that you divorce them from each other as they are different audio formats.

You need an optical splitter.  Toslink and others make them and they are under $10.  With this you can feed an optical signal to the 4 speaker amplifier which now separates it from your Yamaha system.  

If you are feeding a dvd player you can still feed one optical to the yamaha and one to the new 4 speaker system.

The amp and speakers are your selection.

Wishing you well.  Let me know if you have further questions.

PS: I don't make specific equipment recommendations and there are so many subjective factors involved.  Make sure the impedance of the speakers are properly matched to the amplifier; that is an important measure.