In the US, can I buy fish from any reputable fishmonger for ceviche, or should I be using sushi-grade fish?
This is an opinion answer, but and noted in the US "sushi grade" is a marketing label only, not a legal, inspected standard so unless you trust the vendor means very little. Other places it has actual meaning. In all cases you should likely go by what your local standards are, the reputation of the source and your own feeling. Something may be safe, but if it does not feel safe to you, then don't trust it.
In general, my opinion on Ceviche but not backed by research is that it is an "acid cooked" item so is not raw but also has not had heat applied as we know. This says to me that some potential pathogens have been eliminated, but not necessarily all. As long as fresh, quality product is used, the main concern in sushi and ceviche of common proteins is usually parasites. The curing with kill some, but quite possibly not all, so ceviche has less of a worry than sushi, but is not free from fear.
So, my opinion would be to avoid any fish even of high quality which might be prone to parasites, so I would use no fresh water fish. Fish like salmon spend part of their life in fresh, part in salt which makes them more susceptible, so I would tend to use them only if they meet sushi standards. In the US that means hard frozen to kill parasites while in some other places that means well inspected. I would always avoid any farm raised item myself as the crowding and often poor water exchange in the rearing increases risk. Of salt water creatures, I would tend to go for ones which are deeper water, not ones which habit shallow or possible brackish waters which would in my mind increase exposure. If it was an item that seemed more at risk, salmon as an example, then I would want sushi quality only. Mahi Mahi though, a fast growing salt water fish I would just look for good quality from a reputable source. My choices would vary by variety.