I don't really understand what your question is so I'll just answer as I see fit. It depends on what type of text your actual set text is and somewhat on what you're good at analysing. For example, I studied Othello by Shakespeare, which is obviously a play. So, my related texts were a feature article and a film mainly because I'm sucky at visual texts like Leunig. It's advisable that you do a related text that's a different medium from your set text to show your markers that you can analyse an array of texts as opposed to just one particular type.
It's like, if you did Coleridge's poetry as a set text, try not to use Keats' poetry as a related, of course, it doesn't matter if you do, it just looks better if you didn't.