They don't mesh well interms of gameplay, infact they're likely on opposite ends. It's like oil and water. Also even tho DS is pretty violent, it has a very cartoony style.
Gameplay-wise they don't mesh any worse than MK and SF though. And they certainly don't mesh as poorly as SF and Tekken.
And yeah, they are. But I'm pretty sure NRS wouldn't have a problem scaling back to MK3 levels of violence as opposed to MKX levels for a crossover game.