S&P 500 weights are based the value of shares available on the public market not the market cap. Based on that SpaceX will be nowhere near the top 10.

Do you think their valuations wouldn't fall dramatically if they were willing to float a significant proportion of their shares on the market anytime soon.

I addressed that exact point here https://news.ycombinator.com/item?id=48407542

Based on previous IPOs, SpaceX will grow to ~40% float by 12 months, thus if it keeps its current valuation unchanged, will remain near the top 10 spot.