While zeroth does exist in English dictionaries, it’s not something you hear and most natives don’t even know that zero has an ordinal, it was coined in the 19th century up to my knowledge to be used in maths. I’d say the same applies to Arabic and hence it’s safe to say that the MSA term applies.
As for sixth, I can only tell you how I hear it when they speak: it’s essaassa, which has two سين both with a shadda, and the دال is not pronounced clearly (perhaps not at all?). I don’t recall hearing a single Egyptian, including on TV, pronounce it essatta, which is what الساتة implies, but then again, there are several accents even in Cairo alone, so I can’t be positive.