In the US "you people" is not the most polite phrase and needs to be used with care. I would avoid it if I were you. "You are the last people who should talk about _____." or "You are the very last people who should speak on _____" (I would actually use the subject rather than the words "the matter.")

"you people should be the last who speak about that matter" "you people should have the least to say about that matter" These are both more natural. Although, I can understand what you want to say.

It would probably be understood but it it is not technically correct. ''You people should be the last to speak on/about that matter'' is what I would I say. If you wanted to sound extra polished and 'native' then perhaps : ''You people OUGHT TO BE the last to speak on/about that matter'' (since the 'ough' cluster can be pronounced many different ways in English I'll point out that this should be pronounced something like 'orht' )