Some mods are safe to install mid-game, while others aren't. It's more of a problem to remove them mid-game, and even updating mid-game can sometimes be a problem. As far as Izayo's gun mod, I'm pretty sure it will be fine. I had messed up my install of it in my game (I forgot he added a set of subfolders in order to have the optional folders) and so I fixed the install after I was already in my game for a day or two and it worked fine. But I'd definitely recommend making a backup just in case.
Thanks. I backed up saves, and tried it. So far so good. I see the new graphics and guns.
You can use the optional part of the mod to disable the kick. That might help. I used that, so I haven't seen how it works with the helmet light with the kick enabled.
I just discovered that if you hold down "R" for your helmet light mod key, it gives you the choice of light or kick. I'm not sure why that would ever be useful.
R lets you choose which light to activate or deactivate when using multiple light sources - helmet, weapon, vehicle. Kick was probably just added there due to some limitation to how it can work.