the stick push on the 125 came in with the longer fuselages - 800 onwards?
Thence is fuselage length (wing-tailplane distance) a factor in the choice of a ‘T’ tail?
PEI,
The long fuselage came into service with the 600 series 125, the 700 series being essentially a cleaned up 600 but with the Garrett fan engines. These 'stretched' variants had the fuselage length increased
forward of the wing root leading edge (ie the wing - tailplane distance was the same as the earlier variants).
AIRC neither were fitted with a stick push system.
The 800 series and later (800XP, 1000), of course, have a very different (totally redesigned) wing to the earlier aircraft, extended in span by some 4' 4.5" (as well as further fuselage extensions). I would guess that this new wing fundamentally altered the stalling characteristics, hence the stick push system?