How do you know when to use hiragana vs katakana? When reading Japanese text they seem to be intermixed. I read that Katakana is used to write non-Japanese words, as well as used for emphasis like italics in English, but what are the rules regarding this? (I'm sorry if this question is broad)

What does "толка" mean? I know that 'но толка немного' = 'But only a little', so here I assumed it meant something like 'only,'but what does it mean in the context of this song lyric: "Песня музыканта, Санта Лючия Смотри в глаза мои, но только-только не лечи меня Острый, как чили, я сладкий, как мёд пчелиный" And why is it repeated? Just for rhythm? (It is a rap portion so that would make sense)