For the centre, I would really recommend getting the
RSLCR which is designed to work with your RS6 left and right speakers - going for a different speaker (even within the Monitor Audio range) could well affect the tonal balance of the sound. If you must get a smaller centre, I'd recommend finding a Monitor Audio specialist and spend some time demoing the various options they recommend and see which ones work.
For the rears,
these are available or perhaps the
RS1s - again, ideally in the same range, but again if you need smaller, getting the same range isn't so crucial so would go for an audition on what your dealer recommends. Whatever you decide, I would recommend above and all you hear them first to make sure the surround effect isn't compromised.
EDIT - we're like buses here sometimes