I doubt you'll be able to find some comprehensive guideline to the use of contractions in informal writing. You "can" do lots of things—for example, abbreviating please to "pls" or "plz", or your to "ur" or "yur" or "yr".
I wouldn't recommend ever using "’s’s".
In general, I don't see much value in using apostrophes to indicate reduced vowels outside of the standard use of possessive "’s" in words like "witch's". That's the only way to spell possessives like that, so fine, that's the way it is.
But in the case of "is" or "has" vs. "’s", you have a choice, and choosing to use "’s" doesn't really seem useful to me when it doesn't even indicate a reduction in the number of syllables. In a sentence like "It's the best", the apostrophe indicates the elision of a vowel sound—the sentence has a different rhythm from "It is the best," and I think it can sometimes be somewhat useful to be able to indicate that. In contrast, in a sentence like "This's the best", the apostrophe has to be read as representing unstressed [ɪ~ə], so the sentence sounds basically the same read aloud as "This is the best"—most people would automatically use a reduced vowel in "is" anyway unless you used some device like italics to mark that it is stressed.
People reduce vowels in function words all the time, and we don't usually indicate it in our writing—for example, in the previous paragraph, I didn't write "useful t' me when it doesn't even indicate ' reduction in th' number 'f syllables", even though I would pronounce reduced vowels in all of these places.
People do sometimes write and as 'n', but that reduction is a bit more salient than the reduction of [ɪ] to [ɪ~ə]. And in any case, that spelling is I think most common in short titles and so on, not in running text.
As I said, this is just my opinion—people do sometimes use apostrophes in contexts where I find them unnecessary, such as "would've" (pronounced pretty much the same in most contexts as "would have") and "I'd've" (pronounced pretty much the same in most contexts as "I'd have")—but I think most people would agree with me about the unnecessariness of "’s’s" because it just looks so strange, and typing an apostrophe instead of " i" doesn't even save much effort or space.
Somewhat similar: It seems natural to say "who'd've" in speech, but is this incorrect?