Is the Serotta causing pain at this point, or is the issue that the LBS cringed at the fit (I'm assuming you got it from a different LBS than the one that cringed)? Who cares if it measures too small if it's comfortable to you. With your previous back issues, your fit may not be the one you measure for.

Test ride more bikes, different sizes, different proportions. You should have that Goldie-locks feeling of "this one is just right."

I'd be concerned about the Ruby Pro if it hurt right away. That's not a good sign.