tw,
I don't see a way for this to happen, unless slab and beam do not move together.
The cap plate would generally be larger than the tube, so the plate would effectively connect the slab to the beam at that location. Which brings up the question about this apparently being non-composite, and the vertical tube becoming a shear connector...another topic for another thread. At any rate, you probably don't want a path for spilled motor oil and antifreeze to run from under the lift to the area below, so I'd say "no" to expansion material.
As to loading, do not assume equal load on each post - pickup trucks (esp. with diesel engines) are heavier in front. Design for some variation... then factor it. On the other hand, the lift may be designed for 9k/4 on each post, so that might be your live load.
50 psf LL over the lift area plus the lift load is probably overdesign. It is likely that you'd design the slab for the 50 (or whatever), but the structure below would be designed for the greater of either the psf LL or the vehicle weight/lift post load. The weight of the empty lift can probably be called dead load, since it will be "permanently installed equipment."
ASCE 7-02:
Section 4.7.2 might also apply.
Also check note 1 on Table 4-1.
Note that 50 LL changed to 40 LL (in 2002) but the allowable reduction also changed.
IBC Table 1604.3: L/360 LL & L/240 L+D for floors. YMMV