Formal Spanish is well understood in all countries. Slang words may help you sounding more familiar or nice with locals, though there are wide variations between countries. One might come up with some slang terms that may sound nice in one country, while funny or rude in another (even for us Spanish-speakers). Be careful! :D Some examples: "Tubo" means pipe in formal SP, though in Mexico it also means tyre. "Catira" is slang for blonde in Venezuela, while in other places it has no meaning. "Picante" means spicy in formal SP, though it is also a slang for 'sexy' in Bolivia, and 'rude' in Chile. "Joder" means f**king, while in some countries it also emans 'messing around' or 'teasing' LOL

Hello my friend! ¡Hola mi amigo! This is in standard Spanish. ¡Vaya mi pana! Spanish (Puerto Rican) slang.

Depends on what you call basic Spanish. Normally, you could get away with whatever you learnt from a Spanish course, other than perhaps swear words (why don't they include some of the most useful/common words in courses!?). Some Spanish courses (especially those from schools) will have you using the formal "usted" addressing mode all the time though, which is not appropriate for Spain's Spanish. In Spain, you generally address friends, clerks, waiters, neighbours of similar or younger ages, business partners of similar or younger ages, students, and pretty much anybody as "tú". There are very few exceptions, but it's easy to get somebody to think you're talking funny if you use "usted" instead of "tú" than it is to offend an old fogey if you use "tú" instead of "usted". Personally, I don't get offended if addressed as "usted", but I think it funny. I prefer to be addressed as "tú" every time, from everybody, including strangers.