I assume he's referring to http://www.wowhead.com/npc=72113
which will use a cleansing rain/dreadful breath combo on your team. The Gaurdling happens to have both sandstorm and flying attack to use on him but I expect any sandstorm pet should do okay to protect your team.
Yes, exactly this. I normally wouldn't say 'get X pet for X fight', since the whole fun of the Celestial Tournament is to figure out what works. But this is a case where there's a singularly outstanding pet that will become unavailable soon. It would be very frustrating to get to that fight and realize that the pet you want is unobtainable for almost a year.
As Kpb says, there are other Sandstorm pets that will probably do an admirable job, but Guardling has another Flying attack that makes it just a little bit better.
Similarly, P/P is likely to be just a little bit better than the other breeds, for the purposes of making Sandstorm's shield bigger. But again, this is probably not going to be a make-or-break difference. So if you're somebody who has a P/S and is debating getting a P/P, you're probably already well-prepared.