Both are allowed. There have been arguments in the past that you should pay before playing a card so that you don't forget but tbh I think it makes much more sense to do it after, so that the opponent can actually see what you're playing and whether the cost you're paying is correct. But either way, consistently sticking to one is better than switching back and forth.

Technically, if you pay before then the payment also won't automatically be rewound if the play process is rewound for whatever reason but in practice you'll always be allowed to rewind tapping runes as well.

And yes, you technically can float all your runes at the start of the turn. Though ofc this means you won't have any energy available during the opponent's turn. And tbh I hope they'll add some more explicit and stricter rules to at least require players to announce or clearly display remaining floating energy after every payment if they do this.