Maybe @Angelo Farina can answer this. What purpose do those mini fins serve? I was a competitive swimmer and we never used them. Form is vital in competitive swimming and you don't use the same kick with fins as you would without so I would think it's counterproductive to spend pool time using them. You certainly don't need extra accessories for movement specific muscle strengthening, the enormous increase in drag as speed increases means sprint intervals serve that purpose quite well.
You should try them sometime. The difference is the same as doing jumping jacks vs doing jumping jacks with dumbbells in your hands. And yes, the fins you use are very very small so they don't mess up your form: