How do I treat a PMOS compared to an NMOS in small-signal analysis?
The only real difference is in the mobility of the carriers, where electrons are faster than holes (around 2x) thus giving nMOS transistor a 2 times better performance for everything else equal.

About notation, it's mostly a practical thing, and the small signal model is an abstraction that you make to analyze a circuit. So you can use it the way you understand it better.

For instance, I usually use magnitudes (flipping notations drives me crazy) and then determine the direction of the current and voltage from the circuit. So the nMOS will generally have a current from drain to source, and the pMOS from source to drain, both with positive sign.

pMOS's \$I_{SD} \$ will be proportional to \$V_{SG} - |V_T| \$, where nMOS's \$I_{DS} \$ will be proportional to \$V_{GS} - |V_T| \$. Note that for the pMOS, you can flip SG and SD and still obtain the right values, as long as you use the absolute value of Vt.