I didn't want to save my map after messing it up testing this, but I scripted a buff on spawning archons to be .5 size and speed and on spawning eternals to be x2 size and speed, then made a trigger to spawn one of each when clicking dungeon heart. This makes a huge, fast archon and a tiny, slow eternal. If I tell it to make archons large and fast and eternals tiny and slow, the archons will be tiny and slow and the eternals large and fast.
I'm pretty sure the units in party were picked correctly though. I had a parties be default buffed with size and speed and I think I remember the buffs matching the units I chose in that situation.
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