Hi Kieran,
Although you would know vastly more about the NIR system than I would, the following information may be useful. The 900 which had a cab at one end of the vehicle only, has run in formation with the 700s which were cabbed at both ends of the vehicle. BUT supplied 900s to the GNRB in '57/58 to "be run in formations of up to 8 cars in conjunction with the 700s". UTA crested ex-GNR 6car formation Derry-Belfast 1958 had 2x900s at the ends with two unpowered cars and 2x700s, (Irish Railways in Color, A second Glance, Tom Ferris p88)
CIE BUT Green c904n Macmine Jx, 1962 with ex-GNR coach, B'n'T coach and ?4w TPO van p26 same ref.
EDIT: Enterprise often consisted of only a 4 car set on weekdays in winter 133(900)-562-583-121(700),GVS 1968, Norman Johnston's Parting Shot p67
Also 128-594/594-131/134, another 3 car set ? (maybe Belfast-Derry route), p68
123-591/592-121 class also seen alongside, p69
Also 133-572--584, GVS, (1968) p72
I am not sure if these rakes particularly the last are complete or just what was recorded in the shot
Portadown June 1968, 2.30pm GVS-Dublin Enterprise 133-562-552-556-123-125-584-12x?, probably the traditional full set for summer runnning, p81
I have the impression that Class 70 took over about =69, then the Hunslet PPs
K