For so called Oldschool freestyle, meaning jibes, tacks and 360 variations, all not-jumping-manoeuvres, freemove or freeride fins are sufficient. They deliver good performance, a stable ride on the plane and are pretty forgiving when it comes to planing manoeuvres.
When it comes to jumping and sliding freestyle moves, the fin should be as small and rotating as much as possible. This helps in jumped rotations, so the tail doesn't need to be lifted that much and the risk of the edge catching during slides is less.
Extreme freestyle fins are very short and nowadays don't have much area. Lengths between 16 and 20cm can be used for 100 liter boards. These fins do not need much flex and even though they are very short, they should have good performance to be able to run upwind or keep staying upwind.