The point where Nasuverse roleplays diverge from something like this is that pilots are replaceable. It sucks to lose a pilot as they require a certain bond but not to the point where they aren't normally exclusively use to piloting with one person. Servants in Nasuverse roleplays aren't easy to replace. Once they die they can't be resummoned under most circumstances unless the black grail rears its cursed head. Naruto roleplays likewise have a similar thing but depending on the circumstances it may be hard to fit in another character. 

I feel like you're banking too much on the thematical sense of the show, and not the setting when it comes to girl, and boy teams. While it is not a common thing, to my knowledge, in the series that you will find two of the same gender working with one another it may be theoretically possible. Getting an even roleplay count, and keeping it with boy, and girl duo's means that you won't be able to get an even amount of people, or some people won't want to play a girl/boy. It is for the sake of an even roleplay setting that I would consider this to be valid. Spirit of the show put aside you need to make compromises in some cases if you want to make a roleplay.

I don't tend to allow people to make their own roleplay combat scenes however I am well aware of my own lack of talent in the field of combat. I don't go to the arena because of that, and my own distaste for pvp. What I believe the others said is true and that I may need a co-gm to direct combat for the overall sake of a better more enjoyable roleplay otherwise my own lack of creativity may hurt. I don't regard myself as super selective just very wary. If I see something that CAN be abused chances are it will be. I've played enough DnD, and heard enough stories to know that if people can get away with things they will try to.