Colorado,
You're right that the MTH 0-4-0 looks to have had steroids in its past, while the little Porter is among the smallest prototypes I've ever seen. Scaling out most of the On30 equipment, they seem to be based on 2-foot gaugers which were extremely small by design.
You're right that searching out some of the smaller standard gauge trucks from the 2-rail world might be the best bet to make the rolling stock at all believable. But, I think any extensive rebuilding to widen them would be more trouble than just scratchbuilding new larger models. If anything, I would stay away from the full-size pizza cutter 3 rail flanges and go with some older 2-rail wheelsets. Many have an in-between flange size that operates well on 3-rail track and switches.
As far as the Shays, Heislers, or Climaxes are all concerned, they would be real challenges. Just imagining the appearance of a standard roller pickup under one of those gives some idea of the work needed.
But, I'm sure you're up to the challenge. It wouldn't be fun if it were always easy! Looking forward to progress reports. 
Jim
