You are, of course, technically correct. That’s the plural I would use if I were to converse in Latin about the private parts of men I respect.
But for rivals and enemies, I think the use of the diminutive form, “peniculi”, is more appropriate. Although I’m still a bit unsure, as there exist also the female and neuter variants of the diminutive, “peniculae” and “penicula” in the plural, which some proudly male individuals of our species might consider even more insulting.
If you ask me, the diminutive is the single grammatical form whose absence most seriously limits the expressiveness of the English language.