Electronic – General purpose diode between 2 power rails

Those are not Zeners. They are Schottky diodes.

Shottkies have a lower voltage drop than regular Si junction diodes, about 0.3V instead of 0.7V. They are also generally faster.

Their purpose is to keep the inputs within the supply rails.

You should rely on the specifications of your PSU.

The ATX standard has several versions. The power allocation between 3.3 V, 5 V and 12 V rails are very different between older and newer ATX PSU designs.

The original ATX (1995)

The power distribution specification defined that most of the PSU's power should be provided on 5 V and 3.3 V rails, because most of the electronic components (CPU, RAM, chipset, PCI, AGP and ISA cards) used 5 V or 3.3 V for power supply. The 12 V rail was only used by fans and motors of peripheral devices (HDD, FDD, CD-ROM, etc.)

In ATX12V 2.0:

Most power is now provided on 12 V rails. The standard specifies that two independent 12 V rails (12 V2 for the 4 pin connector and 12 V1 for everything else) with independent overcurrent protection are needed to meet the power requirements safely (some very high power PSUs have more than two rails, recommendations for such large PSUs are not given by the standard).

The power on 3.3 V and 5 V rails was significantly reduced.

Update:

In "ATX12V Power Supply Design Guide, v2.31"(It's published in 2008, a little old, but you may take it as a reference), it gives the power distribution of 3.3V/5V and 12V rails from 180W to 450W, and their related max. current limit. In page 69, it gives: for a ATX 450W PSU, the typical 5V+3.3V power < 120W, and 5V rail has a max. curren 15A. .