Sav
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I'll repeat here regarding diplomacy: allow it only for money; if they join for free, the unit must either fight or be allowed to flee. You can't join many good units for money, and this will only provide a way to counter the opponent's Dragon Utopia for stats/control and a severe imbalance in donating to creature banks (which happens often and on any template), when your Dragon Utopia is simply "for money" and you are barely taking them with your own native meat.
p.s. Ideally, even implement (for example) a double price for joining. Want 10 angels - give 40K and sit without money.
Regarding the double price - an option is possible where only half of the squad joins for the price of a full one (as in Heroes 2 with underleveled diplomacy).