Using capacitor to supply temporary surge
You're sort of on the right track, but you can't just put 50V across your capacitor without also putting 50V across your circuit, which would probably damage it.

I would start by saying that you probably want two capacitors, one between +12V and ground, and the other between -12V and ground.

Then, you need to decide how much you can allow each of the 12V buses to "sag" during the current surge. For example, if you could tolerate a 2V drop during the surge, then the capacitance needed would be 0.025 C / 2 V = 0.0125 F, or 12500 &micro;F.

The resistor &mdash; if needed at all &mdash; should be positioned as shown in your first diagram, in series with the source, not in series with the capacitor. But the power source's built-in current limiting will probably be sufficient without the need for another resistor.