OK. Found the answer (I think)
According to specs on
www.yokohama.com.auA 205/50 - 16" will fit on rim 5.5" to 7.5"
225/50 needs 6.0 - 8.0
So hopefully no problems with 205/50's which is reasonable common here in Aust.
Thanks for advice re: sidewall size. i guess the easiest is when i go fo new tyres onto the 6.5" rims, actually go DOWN in width to 205/50's, and accept the speedo out by about 1.5-2%
Cheers!