Is it safe to say that "ii" usually means plural definite? If not, what are the other possibilities?
Yes, it usually means plural definite. But there are a lot of nouns that end in "-ii" in their unarticulated plural form.
copii - copiii (children)
fii - fiii (sons)
scatii - scatiii (siskin)
mii - miile (thousands)
vii - viile (vineyards)
A bunch of nouns (mostly occupation names) who, in their singular form, end in "-iu" (masculine) or "-ie" (feminine):
macaragiu - macaragii - macaragiii (crane operator)
caminoagiu - camionagii - camionagiii (truck driver)
cusurgiu - cusurgii - cusurgiii (a fussy person)
There are also other parts of speech ending in "-ii":
tu știi - you know
fii politicos! - be polite!