[Nikolaguca]

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.

You mean to let the extra hang like in the picture I've attached? Do I gain anything by doing that since I won't be moving the carriages to be below the beam? I intend to use 40mm (1,1/2") thick aluminum riser blocks to connect the beam with carriages and the spacing between the carriages is 10cm (25cm is total length of the riser).

I wanted to make my own mounting blocks since those Chinese blocks use deep-groove ball bearings and I wanted to change them to angular contact anyway. Since I'm doing that I can make the whole block anyway.

Yes, I wanted to tension the screw to remove the sag in the middle. I don't think it takes a lot of force to tension the screw to make a difference, what do you think? A hundred, two hundred pounds of force maybe?

Or is making the plastic support the easiest option? I haven't found any evidence so far that it wouldn't work...

Buying on Aliexpress is not an option for me since the shipping cost is usually way more than the price of the item I was buying. I must shop locally for now but once I gather enough money for my next machine I'll buy proper high quality stuff.

Thanks.