This effect applies before anything that modifies how those tokens enter the battlefield.

If you choose not to apply the replacement effect, you will not get the choice to apply it again until your next turn.

If you choose a legendary creature, the copies will also be legendary. If this results in you controlling more than one legendary permanent with the same name, you'll put all but one of them into their owner's graveyard.

This effect can apply to any token, not just creature tokens. For example, you could replace creating a Treasure token with creating a copy of the chosen creature.

If you create one or more tokens during your turn, and then Esix comes under your control, the replacement effect won't apply to any tokens you create the rest of the turn. You'll have to wait until your next turn.