The middle loco is an American Models S scale RS-3 Diesel that I am converting for use on my On30 layout.
The wheels have been narrowed down from S Gauge to HO Gauge which is what On30 equipment uses. This should confuse you even further.
I don't know if I can help you further in your quest.
Many Steam locos are larger than Diesel locos in real life.
And many modern Diesel locos are much bigger than older Steam locos.
Hope you get all of this sorted out in you mind. You may be over thinking the issue.
For an outdoor layout bigger would be better in my mind.
And the free version seems like a good product (has quite a few brands of track in the library), but limited to 50 pieces of track per drawing. You can always break a plan up into multiple parts and tape them together.