I'm fairly sure the Hiwin manual, available online, gives recommended loading for these rails. From a personal experience perspective, though, you can probably use smaller rails than you would first think, as you are finding from comments above. While I was assembling my welded steel CNC router, at one point I accidentally ended up with the gantry supported at just one end. The gantry is also welded steel and at that point probably weighed around 30kg and all this was hanging off two carriages at one end, on 20mm rails. It dropped maybe 2mm at the far end. A really unfair test of clearances and point loading but it didn't seem to do any long-term damage and persuaded me that worrying about whether I should have gone up a rail size was entirely pointless! Like a lot of builders, though, my Z rails are oversize for their loading - 15mm would be more than adequate but 20mm gives more spacing and only needed small machined recesses to clear the ballnut.

I tapped most of the mounting holes in 6mm steel (3mm wall thickness plus 3mm backing strip) using a spiral-point tap in a handheld cordless drill with a bit of tapping compound. Saved my wrists a lot of strain! I did make a little tapping guide to help keep things square - just a short steel cylinder faced square at both ends with a clearance hole right through. I could hold that with one hand and cordless drill in the other - worked well.