Parallel wall-charging USB-mini devices
If I want to build a 20-device array charger, and each device needs 3-5 Volts of DC, can I take a 5V AC transformer with ~10 Amps, and split the cables 20 times, to get 0.5 Amps per device? I would then solder the power pins on the 20 mini-USB-ports.


Yes. The devices will draw the current they need, not the max current the supply can provide.

Only exception to this would be a perverse device which would use the USB current limit as a constant current source to charge the battery, but that would be a very poor design. It would also probably not work with computer USB ports.

If you're in doubt, get a USB cable, cut the +5V wire, insert amp-meter, and measure current from a standard 5V 1A cellphone charger.