Looking over the RS-42s specs again, they should work just fine for you. I just prefer not to mix so many different size drivers. If you prefer a bookshelf/stereo-sub setup instead of floorstanders, that is great. I prefer that configuration BTW! You could start with one then add the second sub later.
Again I don’t know what your room layout is like, specifically listening position in relation to speaker locations. You could use either the RB-51/61s for the rears or even the RS-42/52s if you have space behind the listening position. One issue with the 51s is that they are rear-ported so they need a little space behind them.