It's more of a "sheng" word, meaning a recent dialect of swahili that is developing mostly in urban areas of East Africa. That means you wouldn't say "poa" in professional or official contexts. While I don't know about it's origins - it may well mean cool in a temperature-sense originally - it is now almost exlusively used to say "good" "fine" etc. There are several ways to say this and they all have slightly different meanings. Somone (in Kenya - might be different in Tanzania) explained it to me like this:
Poa: most commonly used slang
Mzuri: A bit oldfashioned
Fit/fiti: Used to be cool, now sounds a bit "childish"
safi: A bit getto
salama: pretty neutral
sijambo: only in coastal area/tanzania
Sheng is almost exclusively spoken in Nairobi and not at all in Tanzania. Poa is not sheng, even if it is used by people who speak sheng. All those words you listed are used in TZ, except "fiti", at least to my knowledge.
Poa is the most common way to reply to "mambo?" Vipi mambo? and I hardly think it qualifies as slang anymore. It means to "cool down" originally.
But if you're going (let's say) to a job interview, you wouldn't answer "poa" when asked how you were doing, right? At least this is how it was explained to me...
It depends on how they ask...! If they ask "mambo vipi?" I would, but of course if they would ask something with "habari ya...XXX?" then I would answer nzuri/salama. But of course people have different ideas on how relaxed/formal one should be when in different situations and there is no absolute right or wrong here.
Also with young people you can use: mzuka, freshi, mia mia, bomba (literally means a pipe) as an answer to mambo.
I'm pretty sure it's at least cool not hot, but I think it might mean the second too? Not 100% sure tbh. It at least has some sort of idiomatic meaning like that.