Between the 20 and the 40, I would opt for the 20, especially if you plan to go with a subwoofer. The advantage of the 40 is with its lower bass extension, which would come in handy if you plan on listening to some two-channel sources at full range without using your receiver's bass management. But, the 20 has other advantages, and the extra bass with the 40 is less important with a subwoofer.

When Paradigm introduced the Studio v.3 series, the biggest improvement they made was with the imaging accuracy and overall tonal balance. The 20 in particular excels in the imaging, even more so than the 40 IMO. If you plan to eventually use your setup in a 5.1 configuration, then you can perfectly timbre match the mains and surrounds by using Studio 20s all the way around.

Also, the Studio series is not especially difficult to drive, so those Yamaha receivers will work fine. I use an older Yamaha with my Studio v.2 setup, and it gets well beyond tolerable levels before any signs of strain show up.

The salesperson's comments about the wattage are accurate, but that applies to just about every receiver out there. The wattage ratings that the manufacturers quote do not use the "all channels driven" test, so when they claim "100 watts x 7 channels" that does not mean that the continuous output for the receiver equals 700 watts. In recent years, I also believe that the UL standards have become more stringent, so more safeguards are now built into the power supply to obtain the UL listing.

Keep in mind though that a typical speaker will already play at fairly loud levels at your listening position with less than ONE watt feeding into it. The Studio 20 has a sensitivity of about 88 db, which means that from your listening position, you will probably hear about 80-85 db with only one watt of output, depending on your acoustical conditions and distance from the speaker. 85 db is at the high end of most normal listening, and anything above 90 db is really pushing things. Wattage is generally an overemphasized spec that's more about quantity than quality, and it's very easy to monkey around with the spec for marketing purposes.