Prototype 3ft gauge railroads generally used 5" x 7" or 6" by 6" ties, 6 ft long for regular track. Tie spacing was generally 21" or greater. Narrow gauge used proportionately longer ties than standard gauge due to the increased side pressures on the track. Normally switch ties would be set up in 6" increments of length (6 ft 6", 7ft, 7ft 6", etc) until they got to 12ft. Once you needed more than 12ft, you used 2 sets of 6 ft ties again.
I don't know the specifics for 30" prototypes or 2ft prototypes but I believe the 2ft prototypes used around 5ft long ties. I would use 5ft 6" for regular track, and up to double that for turnouts (in 6" increments). On30 is O scale, 30" gauge, so using your O scale rule is correct. HO ties can be used because On30 is the same actual gauge as HO track, but will not look correct. HO scale ties are too short, too narrow, and spaced too closely.
Your buddy doesn't have space for #6 turnouts with only a 6ft run. He will be limited to small cars and engines anyway - a K-27 locomotive in On30 is not small! And a 32ft prototype car is 8.5" over the couplers - that's only 8 of those cars in the length of his layout.
my thoughts, your choices
