
Even if not so actively, we continue these experiments. 🤔
For now, we can say the following:
1) Minis are tied specifically to the affiliation (city) of the creature, i.e., by giving any creature the "Academy City" ability, you can equip it with a Mini.
2) This is not transferred to Upgrades, because of point 1. And even if you have an "Academy" unit that is not upgraded, but the upgrade is no longer "Academy," you cannot put a Mini on the Upgraded unit.
3) Minis are preserved and do not disappear if you "alt-upgrade" or upgrade a creature into a "Non-Academy" one.
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What is the conclusion so far? Based on what is available at the moment, it may be possible to implement Minis for any Faction through an "Alt-racial" unit. But it is difficult to imagine how this can be reasonably integrated into the game.
The experiment also revealed the following amusing fact:

In a city of one Faction, there is nothing preventing you from recruiting units that "by passport" belong to other Factions. (The Neutral icon, in this case, changed automatically because, in this case, the Megeras are "Academy" by origin).
Technically, all this suggests the possibility of creating "domesticated" creature Upgrades, i.e., additional Alt-Upgrades adapted for a Foreign race, capable of using their advantages and not having morale penalties. This may offer some room for experimentation, but it will also require a lot of "dummies" (6 per creature). I am working on the base of "standard" Heroes, without additional Mods, so I cannot get "dummies" for creatures yet. 🙄