you don't really write a piece very consciously thinking about what techniques to use, other than perhaps large techniques that underpin your piece such as motif etc.
instead, techniques are an aftereffect of good writing. it's almost like accidentally using a technique because it suited that part of your piece.
so you normally wouldn't consciously think of using discursive techniques when writing an imaginative, but if it does occur it's fine.
pidgeonholing techniques also isn't the best idea, because as i said, you often unconsciously transfer knowledge from one type of text to another much like other courses where there are interconnections.
something like symbolism could exist in imaginatives, poetry etc.