You might be able to standardize one size axle nut. The axle diameters and threads are likely to be the same.

I don't take group rides a lot. I once went on one and brought my fixie. My friends were amazed to see me descend. Early on, I learned to spin with minimal bouncing on the saddle. It's a useful skill you're not likely to develop without a fixed gear. You will certainly be challenged if you take this bike on a group ride. But that doesn't mean it's not worth doing.

Yes, city life has some pluses and minuses, and they're probably even bigger here in The Big City™. I grew up here and started commuting by bike at age 16 or 17. Also people here have to walk a lot and usually climb the stairs on the subway. The obesity rate seems lower here than in the suburbs. I believe a study showed that bike commuting in polluted air is overall better for your health than a sedentary lifestyle until the pollution reaches a very dangerous level, a level not typical for NYC. I truly do not like the air here but it does not keep me from riding. In June of 2024, we had smoke from wildfires in Québec which turned the sky orange. It was quite upsetting, and everyone had to take big precautions.

Do you describe your home as suburban or urban?