All the pilots from CFE are to be absorbed onto the BA list with effect from a certain date (1 April 2001, I think). Anyone joining after that date, be they a cadet or DEP, will join the list below the CFE guys.
As far as I'm aware, the Brymon and BRAL pilots will not be joining the BA list.
In answer to your second question, the seniority list is exactly that. The longer you have been with the company, the more senior you are.