The MTH tanks look good to me, keep in mind 1/56 to no smaller than 1/43 is larger than 1/48. So I would not put any 1/43 Bradley Fighting Vehicles or Humvees on the same train. Keep any other vehicles you add to 1/50 and you should be fine. The good thing is a M1A1 Abrams is such a BIG tank almost anything you put next to it is going to look small.
The videos are great too. I always enjoy watching them.