I have already replied earlier to the first portion of your statement. Of course I am always available for personal dicussions. Having said that at the outset, let me express my happiness at your interest and the anxiety to know and understand our principles.
Well, regarding the other aspect, this niyamam is not only for Grahastas but also applies to most of the Sanyasis (there again there is an exception for certain type of Sanyasis).
Rishis with long beards, you must have seen illustrations and artistic visuals and came to this conclusion. Long beards are not a must for Rishis and there could have been Rishis without dhaadi too. No dispute over it. Rishis belong to entirely different category. Despite their calibre that was no less than that of the Devas, they are treated as humans since they lived among human beings on earth.
As far this particular subject is concerned let us not enter further into this area now. On Rishis, I shall try to post another write-up with details in due course.
One more point let me add for which I may not be mistaken. For everything let us not search for pramanams and instead just follow our tradition - thats how I was brought up; of course you must be too and several of our generation. For all of us, you are aware, peria vaakku Veda vaakku. For someone for acadamic interest sake it is perfectly alright to go deep into the subject. Let others do the job.
By the by, have you read my other posting on Sandhyavandana?
Sarma Sastrigal