  In order to achieve gauge unification at the string scale M_X sim 5.10^17 GeV
in the minimal supersymmetric standard model (MSSM) it is necessary to add
extra gauge non-singlet representations at an intermediate scale M_I<M_X,
leading to a class of models which we refer to as MSSM+X models. We perform a
detailed analysis of a large class of MSSM+X models and find that the number of
(3,1) representations added must be greater than the total of the number of
(3,2) and (1,2) representations. Predictions of M_I, M_X and alpha(M_X) are
obtained for models with up to 5 extra vector representations than the<EMAIL_ADDRESS>Upper bounds on the U(1) string gauge normalisation k_1 and the sum of the
squares of the hypercharge assignments of the extra matter are also obtained
for the models. We also study the infra-red fixed point behaviour of the top
quark Yukawa coupling in a large class of MSSM+X models and find that the low
energy MSSM quasi-fixed point prediction of the top quark mass is more likely
to be realised in these theories than in the MSSM. In other words the top quark
tends to be heavier in MSSM+X models than in the MSSM. The implementation of a
U(1)_X family symmetry into MSSM+X models to account for the Standard Model
fermion masses is discussed and a particular viable model is presented.
