I would highly recomend pre drilling them. The screws will snap and then they are a real pain to get back out. When I did my trailer I used carriage bolts which turned out nice. If you don't like the rounded tops get plow bolts from a TSC or farm store.
The only reason they tell you not to torch cut it is the slag left on the pipe, grind it off you will be ok. If you want another way, use a grinder and a zip cut wheel once you've marked it with a pipe wrap.
7024 is commonly called "jet rod" and is a flat weld only rod. I agree that the hh187 and the mm210 are the perfect machine for what you want it to do.