Nothing on a cage should be HREW. IMO. The seam can split. Saw it on a lemons car. Had been hit during a race. He had me build a DOM cage.
If you are building a cage for safety, (why else would you build one?) Do not cheap out on material.
Sorry, but I don't lend to this thinking. I totally understand that when looking at the numbers, DOM is a bit stronger than HREW (based on the last step they take for DOM in the production faze), however I believe HREW is plenty strong for the average wheeler out there. Wade likes to go fast and jump a huge truck...which is freaking awesome!! So in cases like that, yes DOM should be mandatory on anyone's list. However, for the recreation wheeler out there that "might" see maybe a slight rollover, maybe two, I think HREW will be plenty. IMO, it's more in the design of the cage to withstand hits rather than the actual material. (and I have heard of HREW seems splitting, but in most cases that's because they put the seam on the outside of bends)
To
@swettysblazer's question on adding HREW to DOM...according to my understanding as long as the thickness of the tubes are the same, it shouldn't be a problem (aka, if the DOM is .120, then make sure the HREW is .120 wall as well). I will say, that looking at your scenario (buying DIY4x kit and then just adding bracing),
@skunked is right that you will probably only need a few sticks and they will all be straight, so the savings isn't as much as if you were building an entire cage from scratch. All I know is doing the numbers on how much tube I used on my cage, if I would have gone DOM, it would have almost been double the cost. And I feel like my HREW cage would hold up just fine. If I had a real bad roll (multiple rolls down the hill), I'm sure the HREW would be destroyed, but I'm confident I would walk away from it. And in that case, the whole truck is destroyed so I would just pull all the good parts off and start on another rig.
Just food for thought from different perspectives is all I'm trying to portrait here