The key line in Bamashooter's response, IMNSHO, is this:
If you do get the anti-rotation pins, get the ones specific to the 15-22 and not for an AR. The 15-22 pins are slightly longer
Many (most? all?) drop-in triggers provide pins that may be too short for the 15-22, or provide pins specifically for the 15-22. For most of the drop in triggers, I believe you can use anti-rotation pins - if you get a set, get one SPECIFICALLY for the 15-22.
Drop in triggers are any trigger and hammer combination where you do not need to mess with the springs - they are internally encased and allow you to 'drop in' the entire group.
Non-drop in triggers (such as the standard hammer/trigger that comes with the 15-22) require a bit more finesse to align the holes for the trigger and hammer.
There have been many reviews of different triggers, and one (perhaps) surprising outcome has been that drop-in triggers tend to have less pre-travel, break crisper, and have less over-travel than their non-drop in counterparts.
Having said that, I run ALG (non-drop in) triggers, CMC drop in triggers, and geiselle non-drop in triggers in my various platforms.