And how? 
I am not talking about any characters in any games, I am talking about the game and only about the game Final Fantasy VII.
It's strange to include characters in a Nintendo fighting game though the main game of the characters wasn't even on a Nintendo console.

Well maybe FFVII Remake is planned for NX. In which case, this makes perfect sense. Personally I think it is coming to both XBox One and NX. If it was PS4 exclusive, Sony would've shouted it from every rooftop at E3 but they didn't; it's not exclusive and if it's not exclusive there's no reason for it to not be on NX. 

I would totally get it on Xbox and wii u. Matters on price of course.

You said it, it might come to NX too but nothing has been announced yet. You have no proof for it to come to NX. So far it is a Playstation game. 
Same thing like Ryu, he is in Smash Bros. too but it has been announced that Street Fighter V will be only playable on PS4 and PC.

Not really, Cloud is more because Cloud= Best SE Rep and is the biggest/most popular FF in the franchise.

If anything he reps all of FF, not FFVII. and FF is huge on Nintendo (obviously more on Sony now, but still. We are talking Nostalgia and FF came from the nes era)

But it's already been explicitly explained that SFV is co-financed by Sony and is exclusive. 

This is not the case with Final Fantasy VII Remake. And really it won't be the case for 99.8% of third party games this gen. Platform exclusives are over because games simply cost too much to make these days that developers can't ignore other platforms anymore. 

NX simply needs to be powerful enough to be able to handle a port and it sounds like it is. Honestly the moment Square-Enix started talking Dragon Quest XI on NX, it likely sealed the deal on their other titles being on NX too. Cloud in Smash is just kind of like icing on that cake. 

Doesn't matter if It comes to NX or not.

As for Ryu, SFV not coming to Nintendo consoles dont mean anything, like Cloud, Ryu reps the most popular/iconic fighting game character. Sonic is Sega, Pacman, etc.

There isnt a rule for Not including a character because of Future/current installments of franchises. Even Banjo Kazooie makes a ton of sense too,  despite BK3 not coming to Nintendo platforms (if it even comes out lol)

But he was in every version of Street Fighter II, that was released on SNES and in the 3DS version of Street Fighter IV.

No.