For a while, I thought that I've been using an APA version of 'Harvard style' but it turned out that these 2 styles are totally different. What are the exact differences and, generally speaking, is mixing between styles a bad practice? (If the intention is for more clarity and the university doesn't ask for a specific style).
Even if your current university and situation do not call for using a specific publication style, I would strongly recommend against mixing two or more styles, even if they are not much different. The reason is pretty clear: consistency. For the sake of readers of your publications as well as for the sake of your own sanity. Following a single style will make your life easier - if you can choose, just pick the one you feel more comfortable with or the one popular, or, perhaps, a standard de facto, in your field (the latter is IMHO much more important - again, that "for the sake of readers" argument).