Well, I've been using my Arteffects Zenith OD infront of my C30 to tighten up the gain and give it the roar that I like. I run the Focus knob at 3 o'clock, the gain cranked and pulled, the treble set flat, bass cranked, and the volume flat.

BUT, when I'm running my SG through it, Feedback is rampaging my sound. Should I get a noise-gate or should I sheild my SG and its 43 year old pickup?
Try running the OD with the gain on zero and the volume full. That helped with my C30 getting feedback from my J&H OD
IT's quite possible that the "43 year-old pickup" in your SG (I assume your SG is a '65, then? If so, pics please) is microphonic. The wax potting in pickups can break down and get loose after a while, causing the copper windings to vibrate. Assuming it was vaccuum potted at all, I'm not sure if they did that in 1965.

A new, modern, wax potted pickup would probably help, but you might want to invest in a new guitar at that point. I'm sure the '65 pickup sounds amazing, it's just not made for high-gain stuff like you're doing.
^ Pickups can be rewaxed potted as well. I had the same issue with modern humbuckers and C30 when boosted, a noise gate mabe an option, or turn your amp volume down/get away from your amp.
Quote by Cachao

Johnbryant you are obviously a genius

My Gear
Custom USA Standard Telecaster
Peavey Triple XXX 212 Combo
Peavey MS412 Cab Celestion G12K-85's
POD 2.0 (the ultimate practice setup)
Guild DV6
It sounds like a baddass, that p-90 roar is optimal for my primordial rock assault. It gives the heavy metal a different tone, grittier, angrier. I am however investing on either a Gibson or Hamer Explorer, or perhaps a Les Paul styled guitar (i'm thinking either saving up for a real gibson LP classic and modding it to john sykes specs, or getting an EC-1000 and doing something simaliar.)

Could any one link me to wax potting threads? also I'm a little clueless as to exactly what that does... I'm going to assume that it keeps the coils stable? Help?