your "front 3" should match manufacturer's series - mainly the tweeters should be the same so the sound does not change as an audio effect moves across the front 3 speakers, from one side to the other.
For movie soundtracks, it is much less important that the side or rear surrounds "match". There is far less audio information in the surrounds tracks and that is mostly ambient and positional effects audio.
My suggestion is get Polk Monitor 50 or 60 towers for the front, a Polk CS1 or CS2 center, and use your R50 towers for the surrounds. Using towers for surrounds means no furniture or stands needed.
