Adding chillies, or chilli powder to the boil. Would that effect fermentation? I hope not coz I've just done it. I can't see how it would but do let me know as I'm fairly new at this...
I usually add ( boiled for15 mins chillies all types from jalapeno to habenaro) to the bottle) to the boil could make a nice hot brew or a blow your head off brew. 1 per bottle allows me to control the heat. But the longer you leave it the hotter it gets.
I regularly brew with chillies (of all types). You can throw them in the boil anywhere from for 1 to 15 minutes (the longer the boil, the more flavour you extract as well as heat). Also, you can make a chili 'tea' - boil (low boil) the chillies a couple of cups or so of water for no less than 5 minutes. You can then add the tea to your already fermented beer (I do it after several days of secondary). That allows you to better control the amount of heat and flavour... taste as you go. Works exceptionally well with dried chillies (I recently made a chili chocolate porter using chili tea from dried Ancho chillies - very interesting). You could also add the chili tea as you're bottling, which opens up even more options for experimenting without screwing an entire batch (learned that the hard way).