No disadvantage to using an AVR for two channel (in fact the AVR has some practical advantages such as bass management, if you use subs). As mentioned, power is not much of a concern. Yes, it will have features you won't use for two channel, but those are not functional disadvantages.
If you want to flesh out your system with more speakers, the discontinued Klipsch Academy is the center speaker that will match your Chorus. You can find those occasionally on the Klipsch forum. Alternately, you could use a Heresy as a center. (Surrounds don't matter as much, but you want the front three to sonically match as close as possible.)