Say the BMI guys wouldn't join the same seniority list, what could be the consequences? British Airways express.....? feeder into Heathrow?
It has been said countless times already that the BACC have already agreed with BA that BMI will be incorporated into BA. The requirements for this (which have been met) were agreed with the IAG board.
If the BMI pilots are not on the BA MSL (unfortunately looking increasingly likely) they will be rostered as a separate group of pilots - they will not form a separate fleet/company. That group of pilots will not see any new members, it will slowly decrease in size with retirement of ex-BMI pilots.