Yes, po' is short for poco (little/few)
For the rules of truncation (troncamento) take a look this site in italian language.
No, in usage, 'a few' and 'few' are complete opposites. 'I have few friends' says I almost have no friends, whereas 'I have a few friends' says I at least have some, and can also mean I have many friends. The first sentence highlights the lack of friends, and the second assures of the presence of some friends, if not many. It's not just a difference in degree of fewness: the sense conveyed is very different.
"few dresses" has very little connection to "some dresses", while "a few dresses" is almost the same as "some dresses", with a very small difference that is usually of no importance. That's why "pochi vestiti" probably shouldn't translate as "a few dresses" - its meaning is so different from "few dresses".
EDITED TO CORRECT ERROR WITH "SUO/SUA" Every once in a while, I forget that DL wants "his/her" for "suo/sua" and put "your" in - and it gets accepted!!!!! I am shocked - not merely surprised but SHOCKED when that happens. And the "suo/sua" isn't even capitalized, "Suo/Sua", which I think it should be for the formal "your".
DL marked wrong "She buys few clothes", marking "clothes" as wrong as suggesting "dresses". That's irritating, since "clothes" is one of the hover-hints. Also, if it were "un uomo compra pochi vestiti", it would be "suits" instead of "dresses". I dare DL to translate this: "Il androgino compra pochi vestiti."