Derailleurs are (with a couple of obscure exceptions) "dumb": they do what the shifters tell them to do. So unless you're using indexed shifters, whether the freewheels have different spacing is irrelevant.
It's you again! Thanks for all your help :)
So I could in theory put a 7 speed freewheel on and use the SL-6400 shifters which are 7 speed and I will be fine?
Derailleurs are (with a couple of obscure exceptions) "dumb": they do what the shifters tell them to do. So unless you're using indexed shifters, whether the freewheels have different spacing is irrelevant.
tl/dr
Yes, a seven speed freewheel will fit fine.