But in the datasheet it says things like: Vds -20V
So if that is how much the drain / source will take before it breaks down you are fine. The negative sign means voltage relative to the source, which in a P-channel FET is sitting up at the +5V level, so 0V will be like -5V to the FET, therefore you are well within the range of allowable voltages. The gate threshold should have a -ve value as well.