DC/DC Converter – Should Grounds of an Isolated DC/DC Converter Be Isolated?

The shielding ground, by which i presume you mean the Earth and chassis as so forth, doesn't necessarily require a low impedance path to the circuit. You could short them together, but if you do make sure its done at a single point, possibly close to the power supply.

You should, however ensure that it's at a fixed voltage with respect to your system ground. A resistor to the system ground shod do the trick. It would hold the chassis at close to the system ground and would also provide a more direct path to earth from any noise or discharge the chassis may experience from the environment, than going through potentially sensitive circuitry.

I am assuming that your system ground is otherwise isolated from the Earth through a transformer.

In general, your analog signals should stay in the analog section of the board and your digital signals should stay in the digital section of the board. You should have separate ground planes for each. However, these ground planes must connect to provide a consistent reference point to both sections.

Ideally, the analog sections and the digital sections should connect at a single point. The "star" connection mentioned in your links and by MoJo. Though sometimes this is not possible. But basically what you want is a small "bridge" to connect the two sections. Any signals that must cross between the sections should be routed over this bridge.

The reason for doing this is to prevent or minimize any noise from switching signals on the digital section from disturbing or corrupting any analog signals that you are trying to generate or measure. The split prevents digital currents from flowing into the analog ground plane. While the "bridge" connects the two ground planes so the ground reference is at a consistent level in both sections.

A good reference for why and how to ground mixed signal systems is Henry Ott's Grounding of Mixed Signal PCBs. Some diagrams from that page will help to explain some best practices:

As for the power supplies, they can come from the same source. Usually the bypass capacitors specified in the datasheets are sufficient to decouple the power between the sections. However, if the noise on the board is particularly bad, sometimes additional filter circuitry is necessary. For instance the one seen here:

Or, you could have a separate regulator to provide clean, stable power to any of your analog devices. This will most likely provide the best accuracy but at extra expense, board area, and circuit complexity.