First aid items are NOT a necessity IMO, and rarely is their likely to be enough room to fit enough in to be particularly useful, beyond an adhesive bandage or two and a few anlagesics. Moreover, since it seems they are called upon to be used more often than the items in a PSK, in my experience, it makes sense to keep those items separate so they are easier to access. For me, a PFAK (Personal First Aid Kit) is an entirely separate item, which itself needs to fit the size constraints of a PSK, and is often even smaller. Bear in mind that given the limitations of a PFAK, due to size, a roll of duct tape in a PSK, along with other elements and normal personal wear, can be improvised to provide virtually all the truly necessary and useful short term medical care a PFAK might provide, except for the analgesics.