No need to create the array at the beginning. It should already exist.
You can also set the size of it beforehand in the preferences over to the left.
Also unless you add a trigger for each xy, it will run every tick, constantly creating objects.
Check out some of the tutorials on sub events.
Other than that it should work. Current x* 32, current y *32 in a 20 x 30 grid should give you 640 width x 960 height layout tho.
Also, also, yes you can nest a for loop, again read up on sub events.