What are the rules governing expressions such as 七八成 (seventy or eighty percent) or 一两个 (one or two)? Must the numerals be monosyllables? What are the restrictions on the measure words? Can there be at most two numerals?
Must the numerals be monosyllables?
In my opinion, you can use multi-sllable numbers, like:
十五六公里 (fifteen or sixteen kilometers)
百八十个 (one hundred or eighty)
三十七八岁 (thirty-seven or thirty-eight years old)
Restrictions on the measure words?
No, I have no idea.
Can there be at most two numerals?
Yes, I have never seen more than 2 numbers used in such a pattern. This pattern is used to express an estimation, an approximation.
I think English usage can serve as a good reference here. In English, saying "7 or 8" sounds normal as a way of expressing an estimate, but saying "7 or 9" does not sound normal as a way of expressing an estimate (instead you would express it as a range of 7 to 9).
Likewise, saying "7, 8, or 9" in English does not sound like an estimate but rather like a specific range or a specific set of choices.
I think the same holds true for Chinese.