Saudi Arabia is only anti-ISIS insofar as they don't want to piss off their Western allies. As I said immediately above, ISIS is heavily Sunni as is the population of SA.
Anyone interested in a long, meandering read of things that led to 9/11 should read Ghost Wars. It's dry, but it's fairly accurate.
True, forgot about the shared ideology.
If any of the M.E. countries with large Sunni populations are finding themselves rooting for ISIS, that definitely does not help, but you won't see any specific action other than underground funding... which is a problem in of itself but not terrible either, at least in the grand scheme of things. They get more money from hostages and private donations.
On the Sunni-Shia divide, I guess we only have Iran, Iraq, and Lebanon to help out (with the Syrian government and Yemen in the mix but not very helpful, for obvious reasons) and Iran will help out slightly because they don't want to see their neighbor Iraq overrun by Sunni extremists now that the Shia majority have the political control of the country. Hezbollah might have something to offer but on a much smaller scale than larger militaries.
But sadly, the Iraqi military is nearly 100% coward, and only the volunteer freedom fighters and the Kurdish Peshmerga actually have any balls to them and are willing to try and take back their land.