Hi Mark,
Although the RX series replaced the RS series, they have quite a different sound. The RS is quite a bit brighter than the RX and subjectively to my ears is more detailed up the top. The bass and mid's both sound different too.
In my opinion, you may be better off trying to find a pair of RS1's on e-bay as then you will have a matching set. If you cannot get hold of the RS1's then also try some of monitor audio's satelite speakers or the RX1. But I strongly recommend trying them at home first. You are at risk of turning a good 7.1 system into slightly muddy sounding 9.1 system by the inclusion of speakers that are not tonally matched.
In terms of partnering the onkyo with the RS series, I am not really able to help you. I believe the amp you mention has a different sonic character to the one I use (905).
This is all just my opinion though.