Re: T-tail aircraft
In terms of CG, rear mounted engines means the main wing is mounted further back in the fuselage, and in most cases, a small rear hold with a small moment arm. Forward of the wings and CG, are bigger holds with a much larger moment arm.
In practical terms; an MD87 needs about 200kg of balast in the nose when empty, but when full, the front hold cannot be fully utilised in terms of weight without cg going off the front of the envelope.
MD81/82/83 were better for some reason even though the long fuselage created bigger moment arms.
Most airliners were designed with a relatively light front SLF cabin for first/business class ops, but high density Lo/Co ops mean that many of 100-150 seaters used need lots of weight in the rear holds.