You can invent any number of nonstandard units, but none of them is "best". It is best to use standard units, that is, the meter.
standard or imperial measurements are in : inches, feet, yards, pounds, gallons and asuming by nonstandard you mean metric measurements, they are in, millimeters, meters, kilometers, kilograms, liters. .........TADA!
yes
Iso means "the same".
The standard unit of length is the meter, and any multiple and submultiple such as kilometer and millimeter. Nonstandard units include foot, inch, mile, light-year, parsec, astronomical unit.
If you mean how many countries have not officially completely changed to the SI units of measure there are 3. The United States, Liberia and Myanmar (Burma).
if you measure a crayon with inches is not the same as measuring with feet.
Handful, chapter, pinch, truckload, pocketful, shovelful, cubit, sprinkle, dusting, 'to taste' are all examples of nonstandardized units. They may describe easily understood 'quantities', but they are in no way likely to mean exactly the same thing to different people.
No in a sense the word uncommon would mean more rare in most contexts, nonstandard, you could say, unconventional, unorthodox, irregular, etc.
The boy was a nonstandard student.
we were using the nonstandard set of measurements
The prefix for standard is "non-" as in "nonstandard."