These two links will take you to some interesting footage of a locomotive that would make a great model. Small enough and big enough.
http://www.youtube.com/watch?v=CnF4jVDpiuc (http://www.youtube.com/watch?v=CnF4jVDpiuc)
http://www.youtube.com/watch?v=85kqHwWQwXA (http://www.youtube.com/watch?v=85kqHwWQwXA)
How about a H.K.Porter 2-6-0? something small to go with the new neat 18' cars.It would also make a great companion with the IF 4-4-0. Use the 2-6-0 for freight and the 4-4-0 for mixed. the Heisler for woods operation and in a pinch stand in for road switching.
Royce
The size is right but its a real plain jane. I think a little eccentric/ eclectic style is a must for an On30 diesel. The styling and odd wheel setup of your 3 truck diesel idea is a much better choice