Levels of your dragons don't matter, they just have to be over Level 4. If you use Breeding Boost you have higher chances, but you don't have to, I got Legendaries 4/7 tries without Breeding Boost, Soccer + Soccer .
Yes and no, so long the rating remains under the 50 this is still the best way to get a legend dragon. Still only when the dragons are lv 4 it's possible to get this rating. Therefore, it's better to get your rating so high as possible. Ones you past 1000 rating it's will have no use to level them any higher.