In my opinion, the only rules which are ironclad are rules number one, five and six. My full given name at birth has all five vowels in the western alphabet, and I am able to successfully lexigram my name, because it has no more than eleven separate, distinct, and different letters of the alphabet. While it is probably desirable to have fewer than all five vowels in the word, name or phrase you are lexigramming, having all five vowels does not necessarily make lexigramming it an exercise in futility. My own addendum to Goodman’s Rules of Lexigramming is:

For successful lexigramancy, it is best if the word, name or phrase you are lexigramming not contain all twenty-six letters in the western alphabet.

In my experience, there are very few words, names or phrases that can’t be lexigrammed. If you come across a word, name or phrase that contains most letters in the western alphabet, then one of two things may be true: either you have on your hands a very gregarious, outgoing, multi-faceted entity that wants to be known, or the subject is employing the defense tactic of camouflage, throwing out a word-salad as a way of avoiding being easily-known. Conversely, if a word, name or phrase has very few letters of the alphabet, then the subject does not wish to be known, and is maintaining its privacy by using as few letters of the alphabet as possible. Further discernment of their character will have to come by other means.

Lexigramancy can be a particularly enlightening practice with words about social issues. To illustrate what I mean, and to demonstrate how lexigramancy works, I have taken for my example the word contraception. Now many people have strong feelings about the subject, both for it and against it. I lexigrammed the word ‘contraception’ and came up with the following words:
CONTRACT-CONTACT-CONCEPTION-TAINT-PRACTICE-PROTECT-PACE-CONTRITE-CANNOT-NO-NOT-TORN-TRITE-TRAIT-TRAIN-TRACE-TRACT-PANT-ACCENT-ACCRETION-CORNICE-ACCEPT-ACE-RACE-ACT-CAN-ONE-CRAP-RACCOON-OTTER-ON-PRAT-PRATE-TIE-TO-TOO-CARTOON-PRANCE-ATRACT-ENTRAIN-IN-IT-RAT-PARENT-AN-OR-ANTI-CONCEPT.