Core 6K is a part of Core 10K. Download Core 10K if you want Core 6K to get the extra cards.
You don't need to "replace" Tango decks with anything. Try both decks, see which you like more.
Core 10K and Tango are quite similar. Both are TSC decks with vocabulary and sentences voiced by a native speaker. You can get them from here. I recommend Tango over Core10k. In Tango the sentences are a little more natural, and the presented vocabulary looks more useful in everyday life. Tango decks have you learn the cards in the 1T order, even though it's not always perfect. Core cards don't follow the 1T principle at all.