Well, if it calls for a cup of miniature marshmallows, we're going to go out on a limb and say you should use a cup of marshmallow cream (fluff). There is little difference between them. Marshmallows have gelatin and corn starch, which makes them set up stiffly. Marshmallow cream gets some of its body from egg whites. But unless your dish is meant to have little chunks of marshmallow here and there, we don't see any reason why you can't make the substitution.

If, after following our humble suggestion, your dish turns out too loose or runny, add a little gelatin to the mix the next time you make it to compensate.