The simple method is modeling a tall wall and letting material priorities do the job -on the typical floor the typical slab just trims the one and only taller module wall by material priority.
A more laborious, all-positive-modeling method is creating a 'bottom wall supplement' on a specific layer (say A-WALL-INTR-supp), and a new layer combo including the supplement layer so as to create two unit types from the same model (publishing combo for ground floor unit includes 'suppl', for typical floor unit doesn't).
Sooner or later some other differences may appear between ground and typical floor units, that justify modeling them separately (perhaps breaking them up into smaller modules, so as to minimize the trace-referencing and repeat manual model changes), but both methods above should be good enough until then.
