Often, when I try to equip more than one magical protection item (ie: cloak and ring), I get the message that I can't wear both. Are there guidelines or rules somewhere that describe what items cannot be worn simultaneously?

Not that I know of, although I do belive that you only can have ONE +1/2/3 etc.
It doesn´t matter if it´s one ring, one cloak, one necklace and so on. But that´s just what I think.

If you carry 2 magical items, the magical aura of them would disturb eachother and the magical effects would become nullified

you have to make a choice :D

anyway..magic doesn't obey 'rules' it flows..

It's always worth trying different combination though as some magical items don't seem to mind being warn with others. Examples of the top of my head (that I'm not 100% sure about) - the red dragon armour, the cloak of the sewers, the Helm of Balduran, etc

So always try mixing whatever you have to get the best combination.

You can also wear a ring of protection +2,ring of gaxx, ring of earth control with red dragon scale armor, helm of balduran and cloak of the sewers