Alright... I've been using carbide tools in set screw tool holders for
years. I've made my own for smaller tools when I was using nonstandard
collet nose spindles for tools that were to small for the spindle. I've
also made my own solid one piece tool holders with a setscrew.

I have to be honest. I've never really had an issue with using carbide
round tools in set screw tool holders, but I am told that set screw tool
holders "should only be used for tools with a flat." I do put the tool
holder in a fixture and tighten the set screw with the maximum tool
pressure I can put on a long arm hex key without stripping the screw
(and sometimes stripping the screw LOL).

So, is it what I have been told true? Should I only use tools with a
flat on them in set screw tool holders? If I do that then am I reduced
to finding a way to put a flat on those tools? How would one go about
that?

I'll probably ignore all that and just keep doing what I am doing, but I
thought I'd ask.

P.S. I use almost exclusively carbide tools except for drills. I even
have some carbide drills in sizes that see a lot of use. I have a few
HSS mills, but I rarely use them.