The first requirement for being admitted to the word list is to be a word. If we were going to accept every prefix signifying a multiple or fraction, we would surely allow "milli" and "centi" - both much more familiar than "pico".
Micro is listed by dictionaries as a word in its own right. It can be used as an adjective meaning very small, or at the most detailed level ("the economic impact at the micro level"). It is also used as an informal term for microcomputer, microprocessor or microbrewery.
Mega is also a word listed in dictionaries, as an informal term for very big. Nano is a casual word for nanotechnology.
As far as I can see the prefix pico- hasn't evolved into a word in its own right, and nor have hecto-, giga-, tera- or femto-, and hence none of them are allowed in Chihuahua.