Could I leave the dough overnight for the first rise as well, or leave it to rise as instructed and place it in fridge over night? (3rd step)

Yes! Make sure you put the dough immediately into the fridge, in a big bowl that is tightly covered. You'll need to build in a bit of extra time in the morning for the to warm up before it really starts into the second rise after shaping. You don't need to wait for the dough to warm up, however, to shape it -- working with it will actually speed up the warming -- but you should plan for extra time for that last rise. ;o)