In case I do not refer to the applicable theory or model in a question which doesn’t explicitly ask me to, will it hurt my marking a lot or will I still be able to cross the passing mark?
If you are asked to use a specific model, but don’t, you will lose many marks.
If you are asked to use a model of your choice and use one that the examiner did not expect you can still score well, but it depends on how suitable your model is.
If the question ssys nothing about a model, the answer would be stronger and more professional if a suitable model was used. However, the emphasis is on ‘suitable’. Don’t assume every question can be squeezed into a model.