Welcome to the wonderful world of Variax and Pod HD! You're going to enjoy the endless possibilities.

Here are my thoughts on your questions:

1) The XPS footswitch is primarily used to provide power to the V600. If you have the later versions of the XPS it also serves as an A/B box to route your outgoing signal differently. But if you have a VDI cable to connect your V600 to the Variax input of the HD500 then you won't need your XPS; the VDI will power yuour V600. You don't need to use the regular jack-output of the V600 in addition to the VDI connection, but you may find an interesting use for it once you get a little more experience; that's part of the endless possibilities.

2) Your instinct is good here - you can use the Guitar In (and also the Aux In if you want to ex[periment further) for your regular guitars and the VDI for your Variax. You can leave all of them connected and use multiple presets and their Inputs to select which guitar(s) you want active at any one time. As for getting the most out of the in and out settings of the HD500, that too is part of experimenting once you gain some experience with your setup. You can come back later with specific questions about that, or browse the forum for detailed information in past discussions.

3) That's largely a personal thing that you will figure out as you go. But there are a few things I can say here:

-  you should set the HD500 Output Mode to STUDIO to feed your mixer. You don't mention using a regular guitar amp; if you do, you should use one of the Live Output modes.

- for studio use it really won't matter whether you use the XLR or 1/4" outputs on the HD500; they both deliver the same signal, and signal quality difference won't really be noticeable over such short distances, imho.

- you will find it really handy to store the Variax model selection in the HD500 preset. That way you don't ever really have to touch the Variax settings; the desired model selection and Tone knob settings are recalled when you select the HD500 preset.