Does the cooling additives have to be adding when you make up your juice before allowing it to steep and mix up together or can it be added after the fact to your finished product in a shake and vape way.
Are there certain flavors that do not work well with them?
What are the max amounts that one should use in a mix
IME Menthol/Koolada are fine to add at any point. The max amount seems to be more subjective than most flavors, though. Some people can go pretty high (over 2%) while others, like myself, find a few drops can overwhelm and entire recipe. Start low and work your way up. That's why you'll see a lot of recipes that call for those ingredients will say "to taste"
I always add Koolada after my juice has steeped. I can then split it into smaller bottles and test each one with 0.25% increments of Koolada until I find the right amount. Each recipe is different of course.
Steeping it is largely unnecessary, as u/Apexified said - add whenever you wish, but for the best effect add it just before you intend to start on the juice as it'll be at its most potent.
I have found that Koolada loses its effectiveness after a short aging and mutes the mix. I don't use Koolada any longer.
Have you noticed this in all mixes? Have you noticed this at all %s? Just curious, I've only ever really used Koolada in drink mixes as "ice".
Nope, no extensive, systematic testing. Just my experience in several different recipes. I've found that I can use Menthol at 0.25% with great results -- nice cooling sensation with just a slight menthol flavor. Koolada, though, at low ratios like 0.5 and 0.6, wasn't giving me the cooling effect of menthol and it "roughened-up" the inhale. It made the vapor very "coarse" on the back of the tongue during initial inhale. I also attribute a muting of flavors to koolada, as, once I took it out of the recipe, I could taste the flavors better. Maybe koolada was doing something to my olfactory system that prevented me from tasting properly... I dunno.
In any case, I don't use koolada any longer, even if recommended in a recipe by a fellow DIYer. I don't think it adds enough dimension to risk blowing a mix.
As far as Menthol/Koolada, I haven't noticed any particular difference with steep times. Mints on the other hand, you may want to let them steep because they can be overpowering at first, and tend to calm down significantly after a steep. Same can be said for Eucalyptus.
Edit: I haven't actually ever used Menthol, because it doesn't necessarily suit my palate, but I would assume its properties are similar to Koolada. If someone can confirm, would be much obliged.