I don't see why not. As long as they pass the "pantyhose test", then it shouldn't matter if you cut them or not. The pantyhose test is a simple way to determine if a fake plant or decoration is too rough for your Betta. Simply rub the pantyhose all over the object in question, and if it snags, even just a little, it is too rough for you Betta. If you cut your plants down, and they pass the pantyhose test, then they are fine for your tank.

 

You can cut them. But you might consider just folding them over at the top. This provides shade for your fish, as well as places to hide and sleep on near the surface.

 

You can cut them unless they are the kind with metal wires in the stems. If so, you don't want to leave the wire exposed to the water as it will rust. A dab of aquarium-safe silicone over the exposed wire will fix this if you really want to cut them.

However, I'd leave them long like Hallyx suggested. Bettas love surface cover, and when I use silk plants I actually buy them a bit taller than my tank to make the fish feel more secure.