You guys are seriously downplaying WB Pirate's strength as a crew. If indeed it is Whitebeard Pirates vs. Shichibukai then it will probably be a close battle.
First off nothing guarentees that all Shichibukai will be there, second only 5 people max will probably be able to fight the WB Pirates due to Moria being out of commission and Kuma being tied up with Kid. Unless Kuma doesn't plan on fighting Kid, I think that they will be in a big brawl due to Kid's power relating to metal somehow and Kuma being a cyborg and all.(loose arguement, but definitely a possibility)
It's more likely that Kuma is going to plow through the Supernovas on his way to the summons. Kuma's being propped up as an uber-power right now, so he wouldn't be matched so soon afterward.
So, if we have all 5 of the Shichibukai fighting WB Pirates, chances are that Whitebeard alone can take on 2 of them, I mean he is the strongest man in the world. Now Luffy has been able to take out 2 Shichibukai by himself, and since Ace is definitely more powerful than Luffy, I'm willing to bet that Marco(1st mate) and Jozu(3rd mate) are able to at least hold their own against a Shichibukai.
First of all, ALL the Strawhats took on Moria at once, and Luffy barely beat Crocodile after Croc decided not to use his main power (until his blind desperate rage at the end), with Luffy on death's door afterward. The Shichibukai are nothing to sneeze at. Coupled with the fact that they have THE strongest swordsman in their ranks (capable of decimating entire SHIPS), as well as a living WMD, and a man who can take control of others, they're not a group to be underestimated. The entire reason the WG has them on their side is their power and infamy, to keep the Emperors quiet. Honestly, with all the Shichibukai, plus presumably the top fighters in the Marines present, Whitebeard is royally fucked if he tries to take back Ace by head-on force. He can have all the fame and titles he wants, with a setup like this, he's screwed.
Based on these numbers alone the WB Pirates are at a disadvantage, but nothing guarentees that all 5 available Shichibukai are going to fight. If all 5 do I'm willing to bet that Shanks will be there to intervene because he cares about Ace. If Shanks crew does get involved you can either say goodbye to Shichibukai, or a retreat/surrendering of Ace due to WG not wanting to disrupt the balance over one pirate.
You seem to be buying into OP's propping-up of certain people a little too strongly. There's a reason that the WG and Shichibukai act as parts of this balance. Do you honestly think this balance would mean dick if 2 out of the 4 Emperors could decimate the WG AND the Shichibukai?! Also, it's very unlikely Shanks will intervene, and if he did, Whitebeard would be royally pissed.
Hell, if Luffy knows better than to try and rescue his own brother, I doubt Shanks would do it.
Now you can counter argue this by saying that 2 yonkou wouldn't be able to take out the Shichibukai, but due to the fact that Admirals make up the other part of the balance, this is definitely a possibility.
The Admirals don't make up the balance, the government does. OP isn't all power levels, there's a political, and yes, "romantic" element to it. As I said in another topic, someone liek Aokiji could probably tear through types like Whitebeard and Shanks (from what we know), but the series props them up to be above everyone else. The Emperors may be powerful and influential, but they're still up against OTHER powerful pirates with a great degree of infamy that outnumber them, and a massive, global network of government and law enforcement with seemingly endless resources, and powerful people in their own rights.