If it sounds good than it is good is a famous quote, and I think its true. But one person might use theory to find what can sound good very quickly, while others struggle for hours to get things to sound "good". You are asking about a countermelody which is usually creating harmonies with the melody, so yes it is usually in the same key. Hiphop isnt above any rules, it is actually much less free than other less popular forms of music. While no one will come to your house if you "break a rule" , no one will think your beat is good if you have melodies in all different keys all clashing. Call it whatever you like, peoples expectations dictate many things about popular music like hiphop.