If it was *my* project, I'd be looking more carefully into why that first servo smoked, and what was going on to cause those current spikes on the servo power rail.
Clamping zeners, supercaps, extra batteries are just band-aids. You need to get to the root of that problem, lest it resurface at flight time. If you have a second set of servos and another power unit, perhaps a bench test with a scope on the power rail is in order? Something's not right there, and it needs to be gotten to the bottom of.
Maybe a clue here?
Not clear if you're powering servos from the 3DM power module, but this page explicitly says not to: