Ah, the peculiar customs of a community that is centered on a fake language. In a conlang community that exists for no particular purpose, except amusement and as an object of study, you can mentally wrap the question "Is it correct to say ...." around all your statements. We have some professors and academians on the board, and from my experience with professors, they are pretty much what one could expect.
If we were working together at a toki pona coffee shop, your coworkers wouldn't correct you very much and you'd naturally improve your grammar over time. Since we don't have joint goal, the content of the message is secondary and what is really of interest is the form of the message-- is it novel, correct, of an interesting style, etc.
As for what counts as right--
Canon -- things that jan Sonja wrote ... well, she didn't write a lot in toki pona, so this strategy which worked so well for Klingon doesn't help toki pona much.
The court of the creator -- in Na'vi, Paul Frommer will respond to requests from fans, but in toki pona, jan Sonja tends not to make rulings, and sometimes used community surveys to gather ideas that may or may not have had much influence on anything anyhow.
jan Pije's lessons -- This is a rewrite of jan Sonja's original lessons, so it is a mix of canon and jan Pije's interpretation. jan Pije's intuitions for the most part are correct and workable, except the bit about "li pi"
the community corpus -- if *no one* or *almost no one* has ever said it that way, then you might be wrong. Or you might be the first to have said it that way.
can you be understood? -- If no one, or most people don't understand it, then it is wrong or at least, well, unintelligible. If I have to re-read it several times, it might be grammatically okay, but it's bad style.
styles -- There is more than one style of writing. the jan Pije and jan Wiko style tends to be minimalistic and favors short sentences that are gross oversimplifications of what ever you are translating. It's like Woody Allan saying, "I speed read War and Peace. It was about Russia." My preferred style is verbose and tries to capture as much of the original meaning as possible and I assume the reader has memorized a lot of set phrases that have been seen in the toki pona corpus. jan Ote is often sort of in between. Another style is sort of conversational and is quick to use sentence fragments, etc. Another style is sort of loglan-like, in that you try to use complete sentences and each sentence follows the rules exactly, almost as if it needed to be machine parseable. In the loglan-like style you sweat bullets over how to talk about indirect speech, how to talk about the sound of an alarm clock (is it mu? or is it a proper modifier?)
rules - There have been a couple formulations of the rules of toki pona grammar, one on wikipedia, jan Kipo's on tp nimi, and one that ... shoot someone posted on the mailing list. They are roughly BNF style grammars, with rules like "a sentence is a noun phrase plus li plus a verb phrase". Grammatical innovation has to be done inside of those constraints otherwise you're expanding on classic toki pona and creating a derivative, which is a good sport, it just needs to be marked as such.