Dipole rear speakers ???

The cancellation issue mentioned by mrrobbcc above does not really exist/come into play from my experiences. You do however want them above your head somewhat ideally. For what you are doing I still like this setup.
Actually, mounted on the ceiling (how high again?) is the better option.
Dipoles are diffuse enough to still be more "non-specific" if you get em up higher. Sure, you ideally want to sit in the null in relation to where the drivers are, but there are compromises. And I still think placing monopoles back there will be a worse choice from an overall sound perspective, in that the sound is booomy and colored, which is the antithesis of proper sound.
The only alternative there is really "Equalization" likely, which is what I'd do, if I didn't do extensive resonators on the wall or whatever. Good luck

Also, you could easily mount your dipoles upside-down,firing toward the floor and ceiling along sides and on the back wall, if need be. Still a better option likely than dirrect firing on the back wall in your sitch, without any other considerations

I have 7/8 year old Martin Logan Quests as sides and as rears. Since the rearward sound of these is as loud as forward,I put carpet tiles around the corner walls that are close by. The result is great (no "bounce, echo, reverb),and they work well with the M-L Prodigy's at L&R . The Quests are no longer expensive.
Hope this helps. Don

I've only used them as side speakers, and they do the job.