Hi John,
Question 1 ...
Quick question about the custom cable, if I plug red cable's banana plug to red on the MINI's output post (and similar for the black connection) ... this will ensure that I get Center positive +
which my Auralic Aries requires ?
This is correct! [Unless you specify center pin negative (which is indeed rare), the cable will be wired for center pin positive]
Question 2 ...
In the future if I decide to use my MINI on a device which requires Center negative - then I would transpose the connection by plug red cable's banana plug to black on the MINI's output post ? I don't have any Center negative - gear (and it seems to much less common), though thought I would just check.
I do NOT recommend you do this. If a center pin negative cable is needed, please specify this when ordering a cable and I will
make the center pin negative when I build the cable. Always connect Red to Red, and Black to Black when connecting the cable to the MINI. Otherwise, it can get confusing and could lead to a mistake, followed by damage (and possibly smoke)!

Question 3 ...
DC outputs on various DC power devices seem to use different types, like XLR, DC , GX16, and for the MINI it has Banana/Spade. Is there a technical reason for using Banana/Spade on the MINI output? Like lower impedance ? I don't mind Banana/Spade (easier for me to solder when I do DIY, I find DC plugs more difficult, so at least one end of the cable will be easier for me to solder).
I decided to use the RED / BLACK binding posts for a few reasons:
1) It's easy for someone to connect their own cable to (even with bare wire connections)
2) There is very little chance of error going with RED = (+), and BLACK = ( - ).
3) Different connections can be used (banana, spade, bare wire or pins)
4) It is what is commonly found on benchtop power supplies.
If I used XLR or some circular connector with a certain pin-out, and one wanted to make their own cable, there is
more of a chance there would be a mistake made when wiring it.
Technically, the binding posts are rated for over 10 amps and I suppose they have less chance of wearing out over
time than something like a barrel plug. The MINI is rugged!

Best regards,
Vinnie