There's one member of our rando club who shows up with a different bike for almost every brevet, including a fat bike, a flat bar 7-speed shopping bike, a 1980s road bike with downtube shifters, a 26" MTB and many others. The only time that I know of that he didn't finish was when a tubular tire of the classic road bike punctured and he couldn't fix it. You can ride almost anything as long as you have the legs for it
And it's true that in randonneuring people largely don't judge you by your equipment choices.
I rode my first 4 randonneuring seasons (including my initial brevet, a 300 km) on a Bike Friday Pocket Rocket folding bike (20" - ETRTO 451 wheel size). While the folding bike had some drawbacks, it was great in many other ways, including low gearing, a dynamo hub and low-trail geometry which besides working well with front loads also means less sensitivity to cross winds. I only switched to my 650B gravel bike (Elephant NFE) after I figured out how to get the same low gearing as on the Bike Friday on a big wheeled bike (with a Sugino OX601D "compact plus" crank in my case).