Bruno, I'm not totally sure I follow. I think what you're saying is that you ahve WordPress SEO (by Yoast) installed currently, and you're wondering if you also need this plugin here installed as well. If so, the answer is no. The plugin being discussed here extends All in One SEO: http://wordpress.org/extend/plugins/all-in-one-seo-pack/ to give it more integration with WooCommerce. So it only does you any good if you're using that SEO plugin, not Yoast's WordPress SEO.
As you can imagine, there's a lot of debate around which SEO plugin is best. I work with some very good SEO guys and they swear by All in One SEO: http://wordpress.org/extend/plugins/all-in-one-seo-pack/. They say there's some serious flaws in Yoast's WordPress SEO that can actually hurt your ranking. I did some research on this (I was using it for a while), and others seemed to confirm the same. For whatever reason, Yoast and Michael Torbert (who created AiO SEO) don't care for each other too much, so you'll mostly only find very negative comments about the other if you go to either of them. However, if you look All in One has been downloaded 12.5 million times. It seems a good bet that if it were causing problems of any sort, we'd probably hear a lot about it around here.
The other factor is that WordPress SEO falls into what I call the "too much" category of plugin. It's kinda too advanced. Sure you might be able to configure it "just right" but you're going to spend a lot of time learning what that is, and maybe hurt your ranking in the meantime. All in One is more of a "set-it-and-forget-it" solution. If you don't put meta tags on individual pages/posts, it'll auto-generate that for you, so that your page/post is still optimized, but without any extra effort. The plugin that's being discussed here, just integrates All in One better with WooCommerce, so that you can add the meta data to each individual product.
Just some thoughts. Hope they're helpful.