I also often think that one should stick to one school of thought, to one teaching etc., but often this leads to narrow mindedness and rigidity. Besides, if a given method works for you better than another, why don't use it?
“Those who teach a Dhamma for the abandoning of passion, for the abandoning of aversion, for the abandoning of delusion — their Dhamma is well-taught”.
(Ājīvaka Sutta; AN 3:72)
Firstly, we can see that the Buddha taught in a manner that is conducive both to oneself and to others. There is often the impression that Dhamma is exclusively concerned for other's, but that's not quite correct:
Whenever you want to do a bodily action, you should reflect on it: 'This bodily action I want to do — would it lead to self-affliction, to the affliction of others, or to both? Would it be an unskillful bodily action, with painful consequences, painful results?' If, on reflection, you know that it would lead to self-affliction, to the affliction of others, or to both; it would be an unskillful bodily action with painful consequences, painful results, then any bodily action of that sort is absolutely unfit for you to do.
There is also the case that some monks during the time of the Buddhs commited suicide because of the asubha bhavana practise. Those monks became disgusted by their own body. This shows you that they weren't ready yet.
Secondly, if we take MN 20 for example we can clearly see that Buddha employed a number of strategies not just one. "If X doesn't work use Y, and if that doesn't work either, use Z".
And lastly, there is the pali word 'upaya', which translates as 'skilful means'. When the Buddha was asked about certain things or teachings the Buddha often gave 'mundane answers' or remained silent, in order to not cause further confusion. For lay people he often began a talk on morality and on the drawbacks of sensuality. Then, when the mind was rid of the hindrances, he gave higher Dhamma.
Similarly, if Mindfulness of Breathing causes you distress then put it aside for a while and do metta meditation.
To summarise, there is no one size fits all. Different people, different history, different dispositions, different preferences & so on.