Title, im farming Pegasus to get Restart but i suspect that the page shows who CAN use it, but any Legendary Duelist can drop it, so i could be farming Rex instead?
That's true, but the thing with non-exclusive skills like Restart or Balance is that although they can be learned from many characters, some certain ones can never be able to learn them. I don't know which characters exactly suffers from this, but judging from my own experience I don't think Rex is one of those characters that can learn Restart.
You use the char with the dropskill you want to have dropped and then farm any LD/PvP to get it.
For example: You want Destiny Draw(Yami Yugi Drop skill) then you play as Yami Yugi and duel any other LD(for example Yami Bakura) to get it. But when you use Yami Bakura, you don't get Destiny Draw as drop when you play against Yami Yugi, as Yami Bakura don't have that skill as dropable. So it doesn't matter who you play against, but who you use as character.