That is also the rule I learned years ago. However, some speakers (it seems to me) make the unstressed o's not directly before the stress have a bit more of an а sound than a schwa sound. Perhaps it is a slight regional difference (if it indeed occurs).

If you do that (turn an unstressed "о" into an "а"-like sound instead of a schwa), you'll sound provincial or like someone from Moscow. In fact, that is one feature that makes the Moscow dialect sound a bit provincial to people from St. Petersburg like me. (That's not just my personal opinion, I could provide links to that extent, but they are in Russian.)

On a personal (and not entirely unbiased) note: if you can hear the difference, you are better off erring on the St. Petersburg side. Think of it as the difference between the UK and US pronunciations (except much, MUCH more subtle): having a British accent in the US is typically a plus, the opposite - not so much.

yes, unstressed о (and а) is pronounced either as /ə/ or as /ɐ/

it is /ɐ/ when it's in the beginning of the word or when it is right before the stressed syllable (e.g. молоко́, облака́)
and /ə/ elsewhere (молоко́, ле́то)

so молоко is pronounced /məɫɐˈko/

while stressed а is usually pronounced like /a/

but I'm pretty sure that most native speakers aren't even aware of these differences, like people just say "unstressed о is pronounced like а" for simplicity (for example I'm a native speaker and I was also surprised when I found out that there is actually a difference)