RP-250S is a dipole design. I’ve never had them and don’t know much about them, so I can’t be much help there.
Yes you can get the surrounds later. It will make a big difference because it will then be surround sound, which is much more immersive than having a front sound stage only.
The main thing in choosing your surrounds is to get them from the same product line as the other speakers so they are timbre matched. All the Reference Premier speakers will have the same tweeters and horns, so they will mesh well together. Other speakers may not integrate well. In that regard, the RP-250S is a good choice. I just can’t tell you if the dipoles are a good idea or not.
Other Klipsch speakers with similar designs might also work, but I couldn’t give a definitive answer on that either. There is a member on this forum who is very experienced with Klipsch speakers, but he hasn’t weighed in here yet.