> Is there a rule or reason which prohibits the placement of a comma between two "simple" nouns joined by {je} ?

In general, the are no hard and fast rules about Klingon pronunciation in general, nor is it known to what extent the punctuation used in romanized canon corresponds to punctuation in pIqaD. Romanized Klingon is described as a transcription system, rather than a transliteration system, as indigenous writing systems were "not yet well understood" at the time that The Klingon Dictionary was authored.

Marc himself has generally not used a lot of commas to separate items in lists joined by {je}; I believe there are examples in canon, but I can't find any.

As a matter of taste, I tend to avoid using commas unless necessary for clarity, because they have a very "English:y" feel to them. That isn't necessarily a problem, as it is a transcription, but in general I want to distance myself from Earth languages as much as possible when writing.

I do use them sometimes when I have a list that includes compound nouns or noun phrases, and/or when I'm composing something that's directed towards beginners.

//loghaD