OK, this is complex, so bear with me Kevin. There's no such thing as EM 21mm or P4 21mm. It's just 21mm. There are different wheel profiles and back to backs for all 3,so to avoid confusion for any other lads, I'll refer to it as 21mm.
The gauge or width of track is 16.5mm, not 16mm for 00. I design all the wagons and locos to 1:1 and then scale them, that's where compromise kicks in. A bit like the taras, you'll have 28mm axles with wheels at 16.5mm.
So, when one approaches the A class for 21mm conversion, it's as straightforward as removing the panel underneath the bogie, removing the axles, (leaving the cogs on the axles), and removing the wheels from the axles, assuming you'll want rp25_88 (EM) or ultrascale wheels.
This is all dependent on the track gauge and scale you have.
As for the queries about colours and details, I know it can be confusing about high waist black and tan crossleys versus high waist EMD's, and cranked livery for rebuilt versions. We will amend the website to make things easier during the week, but we've spent a long time on this project, and all your concerns about wipers, rivets, green livery (which one?) and so on, will all be addressed.
Every variant like sandboxes, wipers, and speedometers have been modelled, I just have to make 15 more models to reflect each variant.
I'm looking forward to the livery discussion.. !
Rich