We investigate the effect of general flavour mixing among squarks on the rare decays (B) over bar --> X(s)gamma, (B) over bar (s) --> mu(+)mu(-) and (B) over bar (s)-B-s mixing beyond the leading order in perturbation theory. We include all large tan beta-enhanced corrections whilst also taking into account the effects of general flavour mixing on the uncorrected quark mass matrix and SU(2)(L) x U(1)gamma breaking. For (B) over bar mu(+)mu(-) and (B) over bar (s)-B-s mixing we find that, in analogy to (B) over bar --> X(s)gamma, there appears a focusing effect which can reduce the contribution due to the delta(RR) (and the delta(LL)) insertion by up to a factor of two at large tan beta and mu > 0. A dependence on delta(LR) and delta(RL), that otherwise ancels to first order in the mass insertion approximation, is also reintroduced. Taking into account the current experimental bounds on DeltaM(Bs) and BR((B) over bar (s) --> mu(+)mu(-)), we find that the insertions delta(RL) and delta(RR) can be significantly constrained compared to bounds obtained from (B) over bar --> X(s)gamma only.