It depends on whether or not they stop at Manors and what length unit they use, but if they're not stopping Manors they could also run into 5 or 7 (or 6 or 8).
The main reason isn’t Manors but conflicts north of Newcastle
Theoretically they can use any platform at Newcastle regardless of stopping pattern with the exclusions of:-
Platforms 9-12 (obvious reasons)
Platform 8 (was only big enough for a 142 if formed from a service coming off the King Edward Bridge, would have to start ahead of the signal if formed off one from the North or the High Level Bridge so wouldn’t probably be planned ).
Potentially Platform 5 but only if it’s a 4 car unit reversing after a service from the HLB / North
Platforms 1 and 2 have direct access to the Down Main and Down Platform** at Manors. You can cross to the Up Main / Up Platform and avoider via Newcastle East Jn (“the Castle)
Platforms 3 and 4 have direct access to the Up Main and Up Platform at Manors however services can access the down line and platform via Dean Street Junction and also to the avoider via Newcastle East Jn
Platforms 5 - 8 (although as mentioned 8 is the least likely) have direct access to the avoider, can cross to the Up Line Platform via Pilgrim Street crossover then can access the Down Line via the crossover at Argyle St Crossover to the north of the station but again this slows southbound traffic
If services don’t use the down main / down platform at Manors there’s going to be more conflicts than just the one from Down Main to the new line at Benton and paths may be pretty tight (it’s only two track from Heaton South. If they use the avoider from 5/7 and not stopping at Manors it’d still have to await a path at Heaton South Junction onto one of the Bi-Di lines then either run “wrong direction” with no further conflicts but block / slow southbound services for a few minutes or go “right direction” and have a full crossover to the Down Main then back again at Benton
** I know Manors is an island platform in case anyone gets annoyed by me saying up / down platform.
I think the platforms are all being designed to take 3 cars but in reality it will probably be a 2 car 158/156 to begin with as that's what Northern has at Heaton.
Pretty sure they are building 100 metre platforms on the new stations leaving the only platforms unable to take 4 cars (92 metres) would be at Manors and the previously mentioned issues with P5 and P8 at Newcastle