PWA has stopped racing supercross, so there is really no need to build a special board just for that. But the Futuras would have done a good job there except that some like single rear strap for supercross.
I don't think the Kombat 96 will be to close to the PA86 - just a sound and health overlap. However, should you want a more spread out quiver, the 105 works too. It demends a bit on what kind of sailing you're doing on the two boards. Since you seem more freestyle and blasting oriented the 105 is interesting since it gives some extra stability for tricks. If you would have been more focused on waves, the 96 would have been the trick.
I don't know about how big a fin you can use in the US-box. There is a 30cm Drake Crossover that definitely work and that is a pretty powerful fin for the size. A more thinner outlined freeride/slalom shape would probably work fine in 34cm.