How can I add some additional 5 volt power
At this point, can I also connect the regulator's VOUT to the Arduino's 5volt out?


Absolutely. But don't daisy-chain the regulator to each supply input in turn, connect each one separately directly from the regulator.


  Also, is it possible (or advisable) to add second voltage regulator to the first in a parallel configuration? 


It is not advisable. Small manufacturing differences between regulators can result in them outputting slightly different voltages, which will lead to destruction of hardware.