Quick update,
Now that I have everything in hand, I decided to not include the planed seconds unit on this clock. I might instead replace it with a small pair of LED seven segment displays. If not that, then perhaps a single LED "ticker" that blinks every second.
Just to be sure, if I put a combined 580mA worth of LEDs and opto-isolators on the I/Os of the MEGA with less then 20mA each on each I/O pin, everything should be fine, right?