I am not sure if there is an authentic book to give you an answer because the question is wrong.
There are no Sudras anymore or rather everyone is a Sudra. Do you know what it means to be a Brahmin. In this thread and in the post below there is a definition of who a Brahmin is according to Gita
None of us make any rules. Only the forum owner makes the rules. Obviously anyone can respond to any posts. But my questions were directed only to those who carry a TB identity in their persona. Only they can answer what makes them a TB. It was intended to be answered from a personal experience...
www.tamilbrahmins.com
Note that the above is all about character and I doubt most people will qualify to be called a Brahmin.
If someone cites something called Manu Smrithi it is not applicable anymore.
Upanayana Samskaram as you call it is only a Phto op ceremony these days. If someone is not doing daily Sandhyavandanam how can they call themselves initiated as Brahmins? I know very few youngsters that are doing daily rituals and live by the character demanded of a Brahmin. So almost all are Sudras. These days they all consume meat and alcohol.
If Veda Mantras are truly words of God (of non human origin I am told) then it cannot discriminate against human beings.
I think some bigoted and narrow minded people have created divisions that are false. Most priests, whose credit is just having memorized large number of Mantras with no clue as to their meaning say 'chant this instead of this'. Almost all have no idea beyond basics to read Sanskrit and have not studied grammar etc,
Yes, I am born in a TB family but that does not give me any special status. I like the Satvic practices of TB culture and trying to re-acquaint myself with all that after spell of long time. But that does not mean any of us can support discrimination between people. I respect my culture I was raised in and I respect all other cultures.
With above set of reasons my suggestion is that if the people you call Sudras want to get married , let them follow their own customs. If they seek the help of priest that person should do all the key rituals as prescribed without any bias thinking in their heart.
Let us all see each other as human beings.
I am not a liberal or conservative and not an authority but using common sense to learn and lead a life..
Please describe motivations for your questions and what the situation is because the question as stated seems to have no real answers.
PS: My apologies because you probably are looking for some authority to come here and give you some answers. Pardon me for exposing issues with assumptions in your questions. I do try to avoid from some people with ego issues - these are usually hypocrites with bad intent regardless of what they call themselves - TB or not TB or or liberal or conservative.