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)
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.
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.
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.