The Stacked Catch stuff is meant to only apply to cases where there is overlap between Toughness powers and Immunity powers, and gives a discount to the immunity to balance out the fact that the creature was already resistant. However, while 'The Catch' is usually presented as an exception to Toughness powers, the first bullet point on YS185 lets you use The Catch to create a limited Toughness Power:
"If your abilities only protect you against something specific, you get a +2 discount. If they protect you against everything except something specific, you get nothing."
Most Catches use the latter half of that for the baseline, then add modifiers based on accessibility and knowledge from the subsequent bullet points, but you'd be starting with a base of +2 since the power works only against "certain forms of energy, including mortal magic, electricity, and fire". You can't get anything from the accessibilty discount, since that is for 'bypass' Catches. You might get additional discount from the knowledge category, but probably only if the power you are creating is a racial/archetypical power of some sort that could be researched.
As to the ideas for Spell Resistance powers, I think that they should actually be bumped up a bit in power. Consider that the Acrobat stunt grants a +1 to defense against ranged attacks (which is probably a more common attack group than spells), and against falls as well. And Powers, since they void the Pure Human bonus, are supposed to be slightly better than stunts. So I'd suggest that the spell resistance either grant refresh cost + 1 bonus to magic defense rolls, or grant refresh cost as a bonus to magic defense rolls and allow the character (once per scene) to spend a Fate point to gain an additional +3.
