Small signal equivalent circuit - MOSFET
For this circuit:  







simulate this circuit &ndash; Schematic created using CircuitLab

The voltage gain is: 

\$ \large \frac{V_{OUT}}{r_x} + g_{m1}*V_{IN} - g_{m2}*(-V_{OUT}) = 0\$ 

Where \$r_x = r_{o1}||r_{o2}\$

$$\frac{V_{OUT}}{V_{IN}} = - \frac{g_{m1}*r_x}{1 + g_{m2}*r_x} = - g_{m1}*\left(r_{o1}||r_{o2}||\frac{1}{g_{m2}}\right) $$

And now let us analysis this circuit: 







simulate this circuit

As you can see I used the N-MOS small-signal equivalent circuit for the P-MOS this time.  

\$ \large \frac{V_{OUT}}{r_x} + g_{m1}*V_{IN} + g_{m2}*V_{OUT} = 0\$ 

And the voltage gain is exactly the same as before. 

\$ \frac{V_{OUT}}{V_{IN}} = - \frac{g_{m1}*r_x}{1 + g_{m2}*r_x} = - g_{m1}*\left(r_{o1}||r_{o2}||\frac{1}{g_{m2}}\right)\$

So to conclude it may sound strange at first glance but P-MOS circuit small signal model is identical to N-MOS. 

We have the same situation with the BJT's 

Why are the current directions in the hybrid-\$\pi\$ model for BJT the same for both NPN and PNP?

Applying hybrid-pi model of an npn-BJT to a pnp BJT in small signal analysis