I have had best success with pepper additions in IPAs, adding about 25% of the peppers during hop whirpool/hop stand, and the remaining 75% during day 2 of active fermentation. I also don't do secondary fermentation, since if I brew an DIPA, it's a NEIPA. I just add the peppers in along with the dry hops, duting day 2 of active fermentation.
If your recipe calls for a secondary fermentation, then you are probably brewing a WC style DIPA, so adding 25% of your peppers to the wort, as soon as you are done with the boil, and are chilling to pitch the yeast. Keep in mind that the more time in hot wort, the more capsaicin will be extracted, and the hotter your beer will be.
The unfortunate part, is that the more contact your peppers have with heat in that wort, the more vegetal/grassy flavors will be imparted into the wort. As you can see, that is a double edged sword. That is why adding 25% of the pepper mass as soon as you turn the burner off, will steep the chiles in hot post boiled wort, and extracting capsaicin, while you chill the wort.
When you add your chiles to secondary, cut them in half, and steep them in vodka, or another high proof alcohol for a few minutes. That will kill off any nasties lurking on the pods, and keep your beer free from infection.
I would personally use aji charapitas for an IPA. They have a good whack of heat, but aso an excellent aroma and flavor, that would compliment a hoppy IPA nicely. Super hots can be tricky to work with in beer, because they can easily ruin a batch of beer, by making it undrinkably spicy.
