Was about to get a toy- then I did some research...

[JAZZCNC]

After much googling and talking to my uncle who's an electrical engineer I think I've got my head around the basics of this. One thing i still find confusing however is that logically you'd want to run the motors at they're rated current but I keep reading that the drives only draw 2/3 of that assuming parallel wiring (hence psu should be sized 2/3*Atot).

Be careful here when talking with electrical engineers ie: Domestic electricians or maintenance electricians because while they know how electricity and how circuits work etc I find they don't always understand or realize how different a Stepper driven machine differs to say typical AC motor system.
A stepper drive uses a chopping system to control the current/voltage that steppers require so it's not straight forward in terms of power draw etc like it is with say an AC motor connected straight to mains voltage.
The drives use a chopping system which uses PWM which only draws current 50% of the cycle on time. This power is taken from the capacitors in the DC system (AC drives just rectify inside the drives to DC) so during the Off cycle time the capacitors are recharging so only drawing power 50% of the time.
This is one of the reasons why the PSU can be sized lower than total Motor ratings. The other reasons being Not all Motors will draw full current all of the time and if they do then it's for very short periods and the Capacitors and drives will deal with any shortfall.

Now, are the drives able to supply the motors the full rated current even if only 2/3 of that is supplied by the psu (through some magic I don't understand)? Or is this 2/3-rule applied simply due to the fact that the motors are seldom simultaneously drawing all of their rated current?

Above should explain this hope fully.! . . . . Don't try to overthink this, I understand the need to understand how it works but if you want to build a good machine then what's been suggested will work great. You could spend weeks or months learning how it all works and you'll still end up back at what's been suggested.
Go with what's proven to work and you won't go wrong.