It's fine and there's nothing non-IGFA about using straight Braid. The only issue is getting braid to consistently test below the line class designation.
WRT Using Straight braid. The lack of stretch MAY result in lost fish due to pulled hooks or broken lines with a fish that head shakes alot, or if you are straight up and down on it in a heaving sea. Any other conditions Lack of stretch in braid can't hurt you or the fish, possibly sudden changes in direction but with a belly in the line, the water provides a level of shock absorption. If you have issues then just go to a softer rod. Doesn't mean go a line class down in rods, it just means get a softer more parabolic version of your line class 15, 24 37kg or whatever line class you are using.
When talking to people who have caught a lot of marlin on light gauge hooks they'll say that LG are more susceptible to breakage on shock loads like heavy strike drags. Once the fish is hooked you can pull as hard as you like. Same applies to using braid because the shock is exacerbated using braid. back off on strike and you're all good.