Dark matter pressure

The answer to your first question, about considering dark matter as a fluid, is that when you are modelling the different components of the universe ('common' matter and dark matter, radiation and dark energy) you work with them as they were perfect fluids, that means no viscosity nor heat conduction.

About the equation of state and the pressure related with this component of the universe I have to say that all these components are studied with barotropic equations of state (the pressure of the fluid only depends on its density):

$$ p=omega rho $$

with $omega$ a constant that depends on the component. The pressure associated to the dark matter is non-zero but it's so small that is possible to consider it as zero.