I picked up a Surly Instigator frame and am getting the parts to build it up right now. Honestly it's the first real build I've organized, so I newbed a couple things and wasn't caught at the LBS right away. First was forgetting brake levers, rectified with a quick call the day before my order went out. Second was the brakes themselves. Here are the relevant details:

Surly Instigator frame + Xtracycle longtail frame
Surly Instigator fork (rigid, suspension corrected)
Front wheel: Shimano IS-6 disc hub coupled to a Sun Ditch Witch, QR skewer
Brakes: 203mm Avid BB7
Brake levers: Avid FR-5
Me: 220lbs + camping gear

I got to looking around online and, lo and behold, turns out 8" rotors on a rigid fork and frame might be a bit much. The price difference was $5 at order, so I figured I'd just go for the extra. I've already got the order in, but I need to amend it to include a couple small items that're likely not in stock.

Should I have them add in a 185mm adaptor and disc, 160mm or try out the 203s? Is the modulation going to be so poor as to throw me over the fork if it doesn't collapse? Will the fork collapse? (It looks like a freaking tank - 3+lbs of steel.) Or will it be fine?