I am hoping for a quick education on the obsolete and forgotten (except by you guys) art of freewheel cogset swapping.

Assuming you stay in one “speed” (e.g. dealing just with 6 speed freewheels), which brands’ cogs are compatible with which other brands? Can you, for example, put Zeus or Regina cogs on a Campagnolo freewheel body? Will Japanese and European bits co-exist?

Now suppose you have various freewheels and loose cogs from mixed “speeds”. Say a jumble of 6 and 7 speed. Can you put the 7 speed cogs onto the 6 speed bodies?

As a practical matter, would you bother to swap cogs anymore? Or is it usually better to simply buy different freewheels?

Basically, I bought an old bike with a 6 speed straight block, and I’m afraid my legs may not measure up. 42/17 low gear is sounding pretty daunting. I could find a larger cogs and swap out the existing 17T cog. Or I could look for a 6 speed freewheel with more humane gearing. Or I could buy a 7 speed freewheel, as I think (?) they will fit the 126mm spacing. The bike has friction shifting.