The problem is that TP doesn't exist in another folder than SMF - it "takes over" the plain "index.php" for itself(which normally is the SMF front) and adds anew section called index.php?action=forum for SMF front.
So having them separate isn't possible as such..but you can do a few tricks, like for example using SSI.php on a page located at your root. its somewhere in these boards, a explanation of that, and it will extract from forum(which can stay at /foro/ folder) and present links that goes into the /foro/ folder then.
About Pagerank..it doesn't mean so much if the site structure changes - as long as the urls that have high pagerank(in your case at the root, without "index.php. Much the same here, I got pagerank 6, but lesser with the index.php attached) stays the same. So i would actually just move everything to root and bet on that pagerank instead, since its higher.