Multiple AC-DC power supplies or use DC-DC converters and one power supply?
Both works, however, the separate AC/DC supply is a bit better.

AC/DC power supplies are dirt cheap.
There are plenty of AC/DC that can be rail mounted, DC/DC is more difficult to find.
You will have overall fewer conversion losses.

Where it is debatable is the 12V -&gt; 5V where you can have a 2$ small converter onboard if you want to save some space, assuming 5V requires low current.
You need to connect all the negative to a common line.