So another follow up.
I used toasted marshmallow flavoring and some vanilla.
850ml Marshmallow flavoring which is about half the recommended rate.
250ml of Vanilla which is my normal dosing for things with fruit.
In hind-sight I should have added less than half the quantity of marshmallow flavoring. probably about 400ml would have been better. The tough part with these flavorings is that they take a while to mix into the beer, so while im dosing and doing my ratio thing, they are very tough to judge. After a carbonating and a full 24 hrs cold and carbed, the flavor is far stronger than i would have preffered.
The beer is not bad, its just sweeter then I personally like. It may mellow as it ages in the kegs.
In the future, assuming that I have time and the need to add odd things into the beer, I will dose it into a Nalgene and let it sit to test the dosing.