It has no problems playing stereo records, although it is useful to turn the selector switch to 2CH. Otherwise some music content might be mistaken for a CD4 carrier and fool the automatic detection.
The CD4 decoder in the 6000 is actually a very good RIAA preamp, probably better than the one in most amplifiers or the builtin preamp in later Beograms. It will of course be approaching its 40th birthday by now, so it might benefit from an overhaul.
I don't think the choice of cartridge affects this at all, although very few will be able to play CD4 records.
But anyway, never even consider converting a 6000 to a stereo deck by removing the decoder!