using a 100pf cap for C6 should be totally fine, the 312 used a 120pf, the 325 used 47pf with a different feedback resistor. You are most likley not to damage the opamp and to be honest, you might not even hear a difference if the opamp is stable.
before you do anything, sleep with this schematic under your pillow for a few nights:
http://www.jensentransformers.com/as/as016.pdf
That is thhe basic input circuit that many many many people are using and you can purchase prefab boards from different people to achieve similar results, if not the exact circuit. this is very simple to wire on perf though and you dont need to use powered relays if you dont want to. Yes, it makes sense on paper but if you are mounting the board inside a shielded chasis, the extra wire run in the box is probably not the end of the world and regular switches with audio running through them are good for years before getting noisy, usually. Relays are expensive and they require power, so its a little more complicated. Ive built many boxes both ways and theres not much difference I can tell coming out the speakers when the record is mixed.
that schem should answer all your questions, but the phantom circuit does come before the pad.
I have a card cage I took off an API console with edge connectors, Im never going to use it, PM me if you can use it.
dave