1. Pushing Limits - Can you only use this skill when the action you want to use is already at its max exertion or can you add it on to one big roll? For example, Batman (melee max exertion level 3) wants to do one big attack, can he roll 4 dice placing one of the energy cubes in his wounds space, or would he only be able to roll 3 dice and then have to do a separate attack of one dice if he wanted to use the pushing limits skill?
2. Shackle- Can a character with the Shackle skill (Penguin’s Gang) elect to do damage with their attack instead of using this skill?
1- You can add it on to one big roll if you want. And fort exeemple with batman you can perform an attack with 2 dice then again an other attack with 2 dice with pushing limit.
2-Yes but you don't use shackle. And as the use of skill are not obligatory it's ok.