72-500s are all freighters and 72-600s are all passenger aircraft with no plans to get either 72-500 pax or 72-600 freighters.
The original plan was 42-500s as Saab 340 replacements where the numbers wouldn't work for more expensive 42-600s on thinner regional routes where the routes (and island airports in some cases) could neither take nor need an ATR72.
Both 42-600s were short-notice additions to the fleet at different times to take up opportunities as they were offered, I believe. Both heading for Glasgow base imminently.
Just btw runway performance and MTOW for the 42-600 is exactly the same as the 42-500 - there's no difference in terms of capability. 18600kg MTOW for both. Some cosmetic differences in the cabin, and quite a big difference in the flightdeck requiring a delta course, training and split OPCs.