That's why I wrote my above post. If the main sweetener in the marshmallow flavor is EM then it won't have a big effect. If the flavor you're mixing already contains it or another sweetener then it'll do f all.have you used ethyl maltol before because i think that is usually the sweetener used in flavours that sweeten like marshmallow
To get a rich sweetness like eg from a ripe mango or a cappuccino you need a different sweetener (sucralose, fructose etc.).
The problem is some marshmallow flavors may have a combination of sweeteners and some may just have EM, it's impossible to tell before buying because they don't put on the label what they used. I don't think there's a general rule like "any marshmallow flavor can be used to substitute for sweetener".