Wow...that's super-annoying! You'd think there'd be some indication of connections/polarity on those.
What you've got with the supply wiring is that the builder used red for BOTH "hot" legs and the black wire should be the common ground. Very dumb...because -12V isn't at all the same as +12V. What I would suggest is this:
Grab your multimeter, first off. If you don't have one, get one...these sorts of issues are precisely why EVERYONE doing electronic music needs one of these on hand. So, what you need to do is to check the pin header continuity, since these are invariably the same on ALL distros. So, start here: http://www.doepfer.de/home_e.htm and go to "Technical Details A-100" where you'll find the pin-out diagrams for the 16-pin standard Eurorack power/bus connection.
Next, check continuity. Put one probe onto the -12V rail pins, then see which of the two red wires it corresponds to. Once you know that, the same pin-out will apply across ALL of the distros. And you'll also know which wires need to be changed out to a different color...I suggest something that you can tell at a glance, such as white. Keep the +12V as red. And yeah...the supply lines from the AC switch shouldn't be changed, as there shouldn't be any DC polarity issues until AFTER the P/S unit.
As for that linear DC honker...that thing has enough current capacity that you'd have to really work at trying to overload it. And like I noted, when the P/S is running way under-spec, it runs cooler, and cooler means better operating conditions for it and most everything else. The stability on it should be rock-solid!