50kg is not an issue at all for two appropriately sized steppers.
200x200 won't use up any travel, just hang the extra out the back. Travel is determined by your bearing spacing which should be decided on based on a compromise - the wider the spacing the better, but the less travel you get from a set rail length.

I think fixed-fixed is too problematic. Only high end commercial machines would be using this. You need very rigid mounting (not standard BK blocks) and the screw needs to be tensioned.

If you are worried about rpm, get higher pitch screws. I use 15mm on one axis and 16mm on the other two.
I use 1616 screws from BST Automation on AliExpress. The quality seemed higher than my previous (multiple) 1605 and 2005 screws.