I wear my EC ninja under mine, it works super well, but the Shiv has a much deeper fit to accommodate a beanie (or facemask) and still fit correctly. This only works if you take out the pads in it. Notice it has a "subliner", which is designed to work with beanie under the helmet. This is one of the few helmets on the market that is actually designed for it. Also notice that it fits prettymuch the same whether you are wearing you ninja (with pads taken out), or if you are just wearing the helmet with the pads in it.
The thing that ISNT safe, in most circumstances is wearing a beanie under a helmet that just sits really high on the top of your head and isnt stable. The Shiv is not that.