The ‘C’ variation is generally for USA/North America, where EMT Conduit is cheap and relatively ubiquitous, and is ~23.5mm OD.
The ‘F’ variation is for Foreign builds, where metric tubing is frequently available, and is for 25mm OD.
And the ‘J’ variation is for Jumbo applications, where 1" (25.4mm) OD is used.
I would say go with the 25mm version, as those slightly smaller tubes are going to be extremely heavy. If you look at the Primo Parts list, it will give the full breakdown (including suggested wall thicknesses).
edit: I didn’t fully read/comprehend your post the first time… You should probably get the other 6 rails recut with the 25mm, but thinner walled stock. Pushing 1.6kg+ of just gantry around (not to mention whatever spindle you add to it) is going to seriously stress your machine (and really slow it down due to acceleration issues).
And welcome to the madhouse! Things can get a little crazy around here (I know, I’m usually the one driving things off the rails), but everyone really wants everyone else to succeed and have fun, so don’t hesitate to ask questions and (more importantly) show us what you’re up to!