Colour (E6 and C41) is easy in that every film is developed for exactly the same amount of time, no matter what ISO it is. I'm not sure if Dwaynes will return re-loadable cassettes, but you can always email them and ask - I think if you put it in the special lab instructions they will though.
When they load the film in their machines, the leader is cup off to a straight edge, so how you cut the leader shouldn't matter at all.