I have both. that is what Dolby expects. I use 3 sets of floorstanders. But if you have one or the other then you should be able to program your AVR to use one or the other.
I guess you just tell it you have only rear speakers when you put them on the side or do you tell it you have side speakers and no rear? I would experiment and see how Dolby adjusts for the missing speakers.
We all have to make compromises.
Jerry