I'm aware that it's spellt with an X, however phonetically it's [ks]. According to the SSP, plosives should come after fricatives word-finally. Does the spelling with an x stop it from violating the SSP? I know English has special circumstances with s regarding the SSP but I thought that was just with [st] and [sp] word initially.
The codas of six and slabs, [ks] and [bz], are in the order "stop-fricative", and in the sequence of features associated with sonority, fricatives are usually said to be more sonorous than stops (it's not the other way around). However, a third option is that stops and fricatives have the same sonority. Admittedly, a sonority plateau is not in conformity with the ideal of rising sonority for onsets and falling sonority for codas, but it's better than a sonority reversal. Notice that English also has [sp, st, sk] onsets but not *[ps,ts,ks]. At the very least, these coda clusters constitute a plateau, so are "outside" the reasoning about sonority and it's relationship to segment ordering.
The coda cluster /bz/ is not actually parallel to /ks/, since /bz/ is always polymorphemic, and the suffixes /-d, -z/ break the general rules regarding possible codas in English.