Reply to "Can I run 2-wheel truck cars and 4-wheel truck cars in the same train?"

I think it would work fine.    You might have the weight issue mentioned above.

Now a question:

Do your original cars have 2 wheel trucks, or just 2 axles?    And do the passenger cars really have 4 wheel trucks, or 4 axle in 2 two rail trucks?   

I am think your originals are like bobber cabooses, simply 2 axles and the passenger have a 2 axle truck at each end.