They have to be the same size or slightly smaller - the "rule" is normally that blades should be at least 80% of the designed diameter, which on a 184mm saw would mean at least 147mm, so 165mm should be fine, but your depth of cut will go down to about 55mm. This is hardly an issue for cement board. More of an issue might be the blade kerf - if you can get a 1.5 to 1.8mm kerf PCD blade for a cordless tool then great, but if your kerf is wider than that watch out for your batteries getting hot, etc. By looking on eBay you might find USA market thin kerf saw blades - Ø184 x 16Ø bore is common in the USA whereas Ø190 x 30Ø bore is more common over here
The 190mm blade might work (I have an old saw which is designed for 156mm blades - but it will take 160mm blades as long as I remove the riving knife, a potential safety issue), but I wouldn't depend on it. So unless you can get an old 190mm blade to try it out I'd probably give it a miss
As to cutting cement board, it is an incredibly dusty thing to do. You need a serious extractor and a good quality mask to undertake this - the last time I did it was with a corded plunge/rail saw (better dust extraction than an open sided/conventional saw), a Festool CTM36 extractor with a pre-filter cyclone, and wearing a JSP Force 8 FFP3 mask (for which I had been face-fit tested). By break time on the first day all I could taste was cement. The job (12 and 15mm sheet) also seems to soak up power, so as I said, your batteries might get hot quite quickly if you are doing a lot and you will find that they don't last as long as they do on wood.
