Someone was telling me that you can Sidestep before Poison goes off and that the reason is because at the beginning of your turn you can choose what order to do any number of free actions: is that true? And if it is can you cite the appropriate rules passage please?

It is true.

From the player's guide, section 1, page 2: "Free actions may be used during the beginning of turn and during the end of turn."

From the rulebook, page 6: "The order in which simultaneous effects are resolved is determined by the active player (the player whose turn it is), followed only when necessary by other players, beginning with the next player and in turn order."

Toss an Outwit in there for some real fun.

If you also have outwit, you can outwit their damage reducer, sidestep next to them and then poison them.

2014 Rulebook, Page 7: Game effects activated by free actions may also be resolved during the “beginning of your turn” or the “end of your turn.”

All "Beginning of Turn" actions are considered simultaneous...

2014 Rulebook, Page 6: The order in which simultaneous effects are resolved is determined by the active player (the player whose turn it is), followed only when necessary by other players, beginning with the next player and in turn order.

-J

For sure. Beginning of the game lasts until you make an action that will result in an action token. Or more specifically a non free action

SLOSH Mano was MVP because of this combo.
Bonus for him destroying walls as a free action then sidestepping through to do penetrating poison.

Oh Harley...

Sidestep, Outwit, Poison. I literally do it 90% of the games that I play with her, and then she STILL doesn't have to worry about action tokens.