It's not true, it all depends how good you are, how good the mic your using is and the location. give us more info on what your doing, and we might help..
You cannot use the K6 to power those. You can plug directly into the camera if you want, you should be able to get usable sound. If you want a portable single, or two channel mixer, then look at the sound devices stuff.
For what you described, I don't think you need a mixer. You might want a mixer, and who am I to speak out against gear acquisition, but I don't think you need one. I'm not familiar with the DVX but from what I've read you have pretty sophisticated controls on it and in an interview setting you don't have to "ride the fader" while recording. Since you are only recording from a single source, why would you want a mixer? The point of a mixer is to "mix" more than one audio source. Now, if you wanted to use outboard gear to process the sound prior to recording it (say a subtle compressor) then that might be a reason, but I would advise against that today. I'm new to video, but have been doing audio work for some time, and these days it seems I do everything inside the box. I would recommend recording the cleanest sound you can on location straight into your DVX (if there are DVX issues I'm unaware of, this advise could be wrong) and then work with the audio in post. I like to bring video and audio into Nuendo (others prefer protools) but you can really do much if not all in FCP (I'm sorry but I'm not familiar with other video editors). Get some good plug-ins by Waves (the best IMO) and you're off to the races.
BTW, I'm assuming that like the XL2, the DVX has phantom power available to it, if not you might need a mixer depending on your mic choice. The other issue is pre-amps. I consider the XL2's pres to be as good as any found on most mixers (I'm not including large expensive studio board in this statement) I'm sure the DVX's pres are of equal caliber. If however you really wanted to use a nice preamp like an Avalon, then that would be a reason to use a mixer and go line in, avoiding the DVX's preamp. For an interview this is not necessary.