PulseAudio Volume Control And/Or Kmix?

I was having trouble quickly selecting between audio output devices (speakers vs. Plantronics wireless headset), so I installed pavucontrol, and after a certain amount of fiddling around, I think I can now choose output locations fairly quickly by opening the PA Control Panel and changing the “fallback” box. However, in the meantime, I still have the Kmix icon in the system tray. Do these two conflict in any way? Should I stop Kmix from autostarting and just stick with PulseAudio?

No.
They are both just volume control frontends to PulseAudio, although KMix supports ALSA and OSS as well.
Changing a volume in KMix should immediately change it in pavucontrol and vice-versa.

Should I stop Kmix from autostarting and just stick with PulseAudio?

You are using PulseAudio in any way.

If you want to use pavucontrol, then there’s no need to stop KMix, but that’s your decision.
There’s also another volume applet for KDE named “veromix”, btw:
http://software.opensuse.org/package/plasmoid-veromix

With KMix you’d have to select the output in KDE’s Audio and Video Settings, on the “Audio Hardware Setup” tab. You can quickly call them by right-clicking on the KMix icon and choosing “Audio Setup”.