I tend to try 7-pools on 2 player maps, but I always lose, because I cannot transition during and out of the rush if it is unsucessful. (Terran already walled off, etc.)
How can I do this?
So, a couple people have mentioned this already, but early pool is very much an aggressive move where you're trading economy for pressure; if you find your unable to apply that pressure that is definitely a disadvantage.
But let's assume you find yourself in this position:
POP QUIZ HOT SHOT
You early pooled only to find a wall off... What, do. You... DO?
So the first thing you'll notice is that you've achieved map control. This means your opponent is going to have to militarize before being able to take his natural. As a result this leaves you in a good position to take your natural. Usually, an early pool is followed by an overlord, so you'll want to expand on 14-16. Expanding earlier is not recommended because you won't be able to take advantage of it.
At this point your goal is to use your Zerglings to "scout" your opponents (ie. run up the ramp and see what shoots at you). What you want to do is get a good feel for what your opponent is building and responding appropriately. This will usually mean either some sort of Roach/Ling play, or teching to quick Banelings.
Ultimately your going to be behind economically, so your goal is to survive the first push by your opponent. If you able to survive that first push, then your second base will start kicking in and you'll rebalanced the economic advantage in your favor.
Alternatively, if your opponent takes too long to push, then use your map control to shut down any expos and expand to a third. The value of a wall is that it allows him to tech earlier to a good early-mid game push (6 minute mark). If he doesn't push at this time, expect some high tech, or cheese play, as he's essentially giving away his advantage.
If you accomplished nothing with it, then you will be behind, and there's no way around that than to hope your opponent doesn't capitalize on it. Buy time and macro hard. Try to keep them in their base, and see if they're teching or just massing low tier units. You can adjust your strategy accordingly.
That said, rushing in 2v2 is only suggested if you both rush so your combined forces have a strong chance to severely weaken an opponent. Dissimilar tactics don't go so well (rush vs. tech/macro).
A 6-pool or 7-pool is close to the textbook example of the all-in strategy. The whole point of the strategy is that you are sacrificing long-term viability for short-term gain. If the gain is not worth it (ie, if you don't immediately win), then you have shot yourself in the foot as far as longevity goes.
I realize that you're trying to mitigate this by having a "plan B," but there is only so much you can do with such an extreme all-in. Therefore my advice is that if you are looking for a rush strategy that can transition into something else, that you start with a rush that is not so far in the "extreme all-in" category. There are other rushes you can do (with later pools, and with other units besides zerglings) which will have much more success transitioning into more economically-sound builds.
More than anything, I can only recommend one thing: Don't do this. 6-7 pools are "strategies" (if they can be called that) that rely on your opponent's failure. This sort of thought behind your build will result in zero improvement on your end, and even if you win enough games to move up in rank you'll eventually reach a level of players who simply won't lose to such rushes.
That said, if you do manage to find an opponent who fails to hold off your rush perfectly, but you can't quite kill him, I suggest a strong, STRONG macro style to catch up. Build exclusively drones and expand, relying on the hopes that your opponent is too crippled to punish such excessive money-making.
Notice how your only hope, is just that: to hope.