Pacifista and pacifismus (both employed by Vicipaedia, by the way) are not exactly the sort of coinage that pleases Latini of the old school in the first place. Too literal, unclassical, would stick out like a sore thumb in a letter of Cicero's.
Now if a revolutionary is novis rebus studens (1 Cat 3), does it not stand to reason that a pacifist is paci studens? This has the advantage that it can be used adjectively as well as substantively.
The downside is that this sort of expression sounds pretty general and does not convey it is referring to a fixed idea. Pacifism is an ideology and a movement (however amorphous) that has a name; being zealous for peace is a character trait. (1) Nevertheless, the adjective pacifist is also often used generically to describe a personal disposition without wanting to make reference to the pacifist movement, and if that is the meaning you are aiming for, you may find paci studens to your liking (or, indeed, you may use other forms of studere).
(1) A particularly egregious example for this issue was the translation for "fascism" I read in a certain book, which was exaggeratum suae gentis studium.