Ice Demons? Is it the Gelugons you refer to? (if so they are devils)
Rewards wise, its pretty clearly Perfidus that hands out more loot, even IF xp is skewed to Desert Trolls. (I don' t believe it is, but it does require less effort and time commitment I grant)
The comment about not everything handing out xp in places has always irked me a bit, its very video gamey to want all inhabitants of a place to fit a CR range neatly. I prefer places be populated with the right type of monsters together and if they are challenging and give xp then so be it, if they are not then they are not. That's my own take anyway. I have always considered it neat to come across the monsters as a reward. If I am level 17 I may not get as much xp from encountering Malebranche or Hamatula as I will Dogai and Pit Fiends but its more interesting to have a variety to me. I just wish that there was some way to interact with and/or learn about these Outsiders and really, all monsters beyond just slaying them. I would be surprised if the majority of the playerbase knows that Malebranche are demoted devils or that Dogai are sometimes known as "Assassin Devils" etc, even if their characters have killed them over and over and over. If there was a server position just to spread readable dialogue or book items throughout the server to ICly share such things I would sign up haha, D&D monsters list are little treats and rewards in a way. I know most probably don't see it that way. Maybe its because I have enjoyed the demonologist dungeon since the days it was on Lake Zarovich and remember my first time in that cave, reading every little scrap of lore I could find on it. I would rather it keep a very diverse Outsider population rather than more of the right CR range for maximum xp gain efficiency. There are some solutions using existing Outsiders that don't require new models, advanced versions of some of the midrange CR types or even add lots of character classes to a Bueroza etc. Shame there is no model for a Paeliryon, that would be a great high CR encounter.