I mix flavours in the tank all the time and have never had a safety problem. If something's not quite right or I just feel like changing it a bit, then I'll put a little bit of something else in there to see what happens. I just did it now as a matter of fact... I'm vaping ecto cooler and decided it needed to be a bit more sour, so I threw in a bit of cranberry (concentrate, not juice - just a few drops) and now I have... I dunno, ecto cranberry cooler or something. Very tasty.
No doubt I'll get bored with that shortly and probably stick something else in the tank as well - maybe pina colada
The only issue I've had is that occasionally I'll get something that tastes revolting. In which case, throw it out, clean the tank throughly and start again. Worst case, I'll waste a couple of ml of juice.