I take it the Rockhopper frame has horizontal dropouts.

When building the wheel using the S/A hub, you should probably use spoke washers that go between the spoke heads and the flange - the old S/A 3-speed hubs have narrow flanges that don't support the spokes very well.

I don't know much about S/A hubs or how easy/hard it is to respace them, and of course you don't want to respace them too far because of increased risk of bending the axle. Hence your stated plan to space the frame down to 126mm. The worry with changing the frame spacing by 9mm is that the dropouts will no longer be parallel to each other, unless you can cold-set them as well (or have a shop do it with special tools).

Given that you're going to eBay trouble and wheelbuilding trouble, I'd say you should just get an older road frame with 120, 124 or 126mm spacing and build the S/A hub into a wheel for that bike. Not too expensive. But that's obviously more cost, time, and space in your garage or wherever.