Many formats are nominal: '6x9 cm' is typically 56x84mm at most, and '2x3' is in the same camp as '3x4' and '6x8', a cavalier disregard for the quarter-inch and half-inch fractions that tell the rest of the story.
External dimensions of the cut-film holders for 2-1/4 x 3-1/4 inch, 2-1/2 x 3-1/2 inch and 6.5x9cm are identical: only the internal dimensions vary. Personally, having tried it, I see little attraction in piddling about with small bits of cut film: I'd go for roll-film every time.
Tech IV and later 'baby' Linhofs have an 'easy-on, easy-off' back with a single movement: earlier models have four wildly inconvenient fingernail-breaking slides. Both accept the same Linhof-unique rotating fitting.
As already noted, the most common roll film back is 10-on-120, 56x72mm, which enlarged 3x is whole-plate (and can pass for a contact print). Yes, 6x9cm backs (8-on, 56x84mm) are available -- indeed, they are still available new, I believe -- but they are rarer and more expensive on the used market. My old (60s/70s) Super Rollex 6x7s wind on with a single movement: my wife's late 90s version, new, Alpa-modified, requires multiple strokes of the lever. Like everyone else, I'd recommend Super Rollexes over plain knob-wind Rollexes.
You can often (though not always) have old cams peened and re-cut: Bill Orford (
www.photographicrepairs.co.uk) has done this for me more than once.