Basically any machine tools are subject to rust, and it is advisable to keep steel surfaces coated with oil. I simply use a spray oil & wipe the rails. I keep my shop heated to well above the dew point temp. Before I turned my heater on for the winter months, I went out into my shop to find my mill table wet with dew. Had I not caught it quick, it certainly would have rusted. FYI, the area of rust on the bearings that the user from South Africa showed is not a critical area of the bearing, but it could definitely benefit from some oil before it gets any worse.
The rails are VERY strong, and no, they’re not going to flex. The rails are 35mm O.D. x 23mm I.D., so 6mm wall thickness. It would take magnitudes more force than a router cutting wood or soft metal is capable of applying to the rails to cause any measurable flex.