I am also using Mustela product, I tried the Barrier Cream before but only when baby under 2 or below. You have to use a lot to make it become a barrier to the urine and ... that is why you can't use it on a ready- to-run baby.
I agree it is not necessary to use steroid, try the Ego brand for baby rash. But if the rash is growth use a bit of steroid topically would not do much harm.
This is my personal experience only.