You might try adding an additional custom one of each type and/or be explicit about the regular Council ones that are part of the group, rather than using a "random Council minion" option, which I assume is what you use now?
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It's possible that the game does a first pass through on your spawn list, looking for "random", and if it finds it, to just do the random and ignore everything else.
I can confirm that mixing customs and specific dev mobs together does what you're expecting it to do. Just pick and choose the Council baddies you want, remembering to cover your entire desired level range. Some authors do a similar thing to remove particularly overpowered/underrewarding dev mobs from custom groups.
I made a custom group which consists of the following:
MINION
My Custom Mob (Level 1-54)
Council - Random Minion (Level 1-54)
LIEUTENANT
My Custom Mob (Level 1-54)
Council - Random Lieutenant (Level 1-54)
BOSS
My Custom Mob (Level 1-54)
Council - Random Boss (Level 1-54)
The idea is that there's a new kind of Council soldier and they're mixed in with the rest of the usual suspects. The custom group's name is "Council."
However, when I ask AE to populate a map with this custom group, it only spawns the standard, game-made mobs, not my custom ones. I've tried looking at it with x8 difficulty, and even in these huge groups not one of my customs appeared.
Oddly enough, when I had an ambush spawn from my Custom Group, they DID spawn in the ambush.
I'm mystified. I have treble-checked to make sure it's the CUSTOM Council group set as the enemy and not the Standard one. Any thoughts or help on this issue?
Thanks in advance.