> No the float rule is to avoid having to buy so much stock compared to the available stock that it would create irrational prices

Correct.

> this is probably going to happen with those IPOs

Not due to any index-following investor.

> SP500 can decide absolutely whatever they want

Yup, S&P 500 is a committee-based index.

> one of those instances in which they changed the rules in a way that made no clear sense and they will be remembered for that

S&P never changed the S&P 500's rules.

NASDAQ 100 did. But from what I can tell, that was a brilliant piece of marketing. Nobody talked about them before. (QQQQ doesn't appear to have gained or lost net assets in that time, which isn't unexpected, it's a volatile fund.)