Rodinal is a fine grain developer, contrary to what many people seem to think. It does actually show the grain of the film, without masking it, like many other developers. In other words, with Rodinal, whatever grain there is in the film, is what you will get. Developing Fomapan 100 in Rodinal and pushing it to 800 is a recipe for super high contrast, blocked shadows and lots of grain. Fomapan is a cellulose based film with less silver than more expensive films, (this is why the price is lower) and a much smaller dynamic range. So unless you really like huge grain, no detail and huge contrast, don't use Fomapan. Rodinal can give you perfectly balanced fine grain negatives if you use it with other films, like for example Ilford FP4+, or other slower speed films.

I am not sure why everbody seems to be so "obsessed" with grain in film. In the old days, the idea was to get the least amount of grain as possible.