For Ancient Forest, during a duel, there's an option about auto chain building. Disabling that should fix the conflict between Forest and Gladiators. (Sorry if this isn't clear enough, I don't have access to the game at the moment >.< The options panel should be on the left, where the card descriptions are.)
Tragoedia has ? original ATK, which is treated as 0 for card effects while on the field. Shrink would halve that (half of 0 is still 0), then his effect would reapply and boost him by 600 for each card in your hand. So basically, Shrink does nothing to him.