There is flex in all types of unsupported rail... be it 12, 16 or 20. How much in these I don't really know.

It all comes down to how big you want your router and what types of material you want to work with.

You won't cut the shafts with a hack saw or anything like that. I tried once for about 5 mins and left a barely visible mark which I coulden't even feel when I ran finger over it. Sometimes the rails have kind of scorch marks on the end, so I am assuming some sort of heated cutting is involved.

For all that they cost its best to order them the length you need.