Perhaps a tail ring could substitute the magic boots and fill the "boot" slot. Sound plausible and easy. This is magic we are talking about.
Oh absolutely as a GM I would allow Merfolks to use magic items that go on their tail/boot slot. I would not allow boots though. It would have to be a non-standard magically item designed for tailfins which I assume Merfolk magical crafters would have developed long ago.
I was only discussing academically what the rules as written state.