From ReVo I know that creationism is kreismo in Esperanto. Is then kreismulo a person who believes in creationism?
If kreismo is creationism (which I would agree with) then, creationist would be kreismano, as a creationist is a member of creationism.
I think kreismulo (or even kreismisto) is a safer choice, as kreisto also means "creator" and I'm not sure I would understand it correctly as referring to a follower of a belief system.
An item in Monato uses kreismemulo. I would interpret that as "sympathizer with Creationism".
In nearly every case, -ismo transforms regularly to -isto*. That is, if the starting point is kreismo, then someone who follows this school of thought is a kreisto.
It is of course possible to form an expression ending in ismano, but I have primarily seen this when the isto form is taken for something else - for example, Esperantismano is used for members of Esperantismo, since Esperantisto is defined simply as Esperanto speaker (regardless of the purpose they use the language for.)
I can't say that I have ever heard the word kreismo used, but a little bit of poking around the web suggests that it has at least some traction. Any objection to kreisto with this sense would apply equally to kreismo with this sense.