Never tried this myself so not sure how it'll pan out
Given such a shallow map you probably want to disable or reduce caverns.
There is an option for number of cavern layers: 'Cavern Layer Number' Accepts 0-5 with 3 being default
You are probably also wanting to tweak the following settings to get your shallow world:
'Maximum Elevation' - This defines how high land can get and supresses things like mountains. Bear in mind dwarven civs start in unsavage mountains, you need at least enough to get a dwarf civ in or the world wont be playable (rejected)
'Elevation Y variance' - I think stops the elevation from fluctuating so much, lowering the value above may render this fairly moot
'Z levels above xxxxxx' - there are 6 of these variables, with the x's denoting ground and layers 1-5, from my understanding these dictate the amount of z levels between certain underground features. 5 is HFS, 4 magma sea, 3-1 are the various caverns and ground is...well...the ground. im not certain how the values you put in here dictate what actually appears, the defaults are small values, much smaller than the # of z-levels you will actually see
Overall i've not been hugely helpful, I dont know what values you need to generate the world you want, however the variables i listed are likely the ones involved. It may in the longrun be easier to attempt to sculpt something in PerfectWorld