Late second pitch to dry out a porter?

I am currently on day 7 of my second batch of beer which is a porter based on a recipe from my lhbs. I took a hydrometer sample today and tasted the sample.
The beer is currently at .02 and its supposed to have a final of .015, but the real problem came in when I tasted it. It tastes very sweet, closer to an American style porter, but I prefer a very dry porter.

At the time I didn't know much about yeast and used the yeast recommended in the recipe which was Wyeast 1332. I now look at the 1332's attenuation which is 67-71%. I should have gone with a British Ale with an attenuation closer to 75%, but my question is can I do anything to dry the porter out? Say, pitch some English Ale yeast and let it sit some more in the fermenter, or will that just mess with the beer?

And for the future, where is a good place to learn more about yeast and how exactly they will effect the taste of my beer?
Thanks

should be able to, but youll want to consider the correct amount. start a beer in mrmalty at 1020 and it should recommend only a fraction of say a 5 gram dry yeast packet to finish the beer off.

Yeast by white and zainasheff is a pretty complete resource for all things yeast.

You'll want to make a starter and pitch it when it's really chugging.