1) IS350 wheels, yes. IS300 (meaning 01-05 model years), probably not
2) No. even stock they're staggered: 225 width in front, 255 width rear
3) No. rims come in different diameters, widths, and offsets.
Any problems that may arise would be due to going to different sized wheels and/or tires that don't have proper fitment (meaning may stick out too much, may not stick out enough, may be greater or lesser rolling diameter than stock). Wheel width and offset greatly affect fitment. Diameter not as much (assuming you keep same outside diameter meaning adjust tire size accordingly)