First of all, yes, the HF-G25 is essentially the same as an XA10 minus the XLR audio inputs (not a problem if another camera will take the stage audio feed – ideally not the DSLR as they have awful audio circuitry and no balanced inputs) and with a slightly better (20% Canon say) sensor.
The Panasonic x920 is better still in low light however, though still not great.
The trouble is that none of these small camcorders will be very good for you. For a start you’re trying to film the most difficult thing to shoot which is a stage show. Inevitable they’re a hotch-potch of really bright and really dark areas, giving huge contrast range which is not something that any camcorder copes very well with, even the broadcast ones let alone a consumer unit.
It is unclear from your email whether the additional cameras will be manned or set up once and left unattended. If the former, then the HFG25 is a better choice since it has slightly better manual controls (namely a separate iris control thumbwheel) whereas the Panasonic is all about fiddling with on screen menus which is not ideal in a “real time” situation.
If left unattended in auto mode then either camera will be all over the place for exposure as they try to cope with the extremes; your other option is to set them once in manual mode and accept that some of the scenes will be captured correctly while others will be overblown or too dark.
Whichever you do, the dark spots will be very grainy and noisy due to the small sensors used in these cameras.
For all the ergonomic awkwardness of a DSLR (and they are not ergonomic for video, that’s for sure), they do have the advantage of much better low-light capabilities and higher dynamic range. You may be better off with a couple of low-cost DSLRs or bridge cameras which you can set and forget.
You mention recording direct to hard disc from the main camera. For this you’d need capture card that takes HDMI output from the camera (and you must make sure your DSLR provides a CLEAN (no on screen menus) HDMI output at full resolution – not all do. Have you considered standalone recorders such as the Atomos Ninja 2 or Ninja Star?
It’s unclear how you propose to take the audio mixer output into your system for recording but if it’s XLR balanced out (and I’d advise that’s what you take to minimise electrical noise pickup), you’ll need a suitable recorder to tape that sound – something like a Tascam DR60D, which I believe they’re about to release a new version of – which you can then synchronise to the DSLR’s poor-quality internal track during the edit.