Can I substitute red onion for green onion
Red onion, or any other onion for that matter, is a pretty reasonable substitution for the white portion (on the right in the picture below) of the green onion/scallion.  They're all alliums, after all, and have a reasonably comparable flavor.  As @Stephie notes in the comments, you may want to use a slightly smaller portion; red onion tends to be a little more pungent.  Another way that you can deal with this pungency is to soak your diced red onion in water for a few minutes, which will dilute the heat. (Which works for white onions too, by the way.)

The green portion of the scallion (on the left) is a bit grassier in flavor and much less pungent; red onion isn't quite as good a match for this part.  If you have access to them, chives would be a better replacement, unless you're cooking the dish, in which case chives would lose a lot of their delicate flavor.



Given the other strong flavors you've listed, red onion will probably do just fine. I'm guessing that the onion is a relatively small amount at any rate, and just there to add a bit of textural contrast and a little extra sharpness.