Looking at the traces, the areas circled in green all seem to be connected (well, within their own board). Can the bottom connection points be used as ground connections as well, or just the top?
How are you running them, each powered with a separate winding?
If so, keep them separate at the inputs. Only connect them together at the outputs,
which will be between plus and minus (GND 0/1/2 and GND 3/4/5) to make a bipolar supply.
So the green-circled connections, not the red-circled? What's the difference between those two?
Also; I've read that it is best practice to run the biggest ground wires feasible. Is there any reason not to run wires to all six ground connections on each board?
Whichever pad is closest to the wide bus area they are connected to, is the best one to use.
There's no reason to use more than one wire though. It's a matter of getting the least noise.
