I will take a shot at some of your questions.
There was a fella who used to post about what radius his MTH locomotives would make. I believe he used to say that the MTH Big Boy could handle a tighter radius than 60” but I am not 100% sure.
MTH scale wheel versions— yes, you do need a special version. This I am sure. The version you need is noted on the part number by having a “-2” at the end of the part number. With the “-2” you will get scale wheels and fixed pilots. This I am sure.
Good luck.