It's simple that we don't want everyone being good at everything, so there has to be rules and Limitations.
1. If you do not have kenjutsu as a specialty you cannot have a special sword over C-rank.
2. While other weapons are allowed, please be reasonable and fair.
3. A sword used by you cannot be a higher rank than you are I.e genin should not have A-rank swords, You can always update them in the future.
4. Everything can break, I don't want to see "It is made from an unbreakable metal" and so on. It may be tough but it is posible.
5. Any elemental, swords must be an element your character uses
6. No gravity swords...it happens
7. While cannon swords such as zabuza's decapitation swords are allowed, they are completely cannon. You are free to make your own version but well, lets keep things simple for things.
8. To have an S-rank weapon you must be a swordsmen of the mist.