I'll repeat here regarding diplomacy: allow it only for money; if they join for free, you must either fight or let them escape. You can't join many good things for money, and this will only provide a way to counter the opponent's Dragon Utopia for stats/control and a strong imbalance in donating to creature banks (which happens often on any map template), when your Dragon Utopia is simply "for money" and you are struggling to take them using only own native units.

p.s. Ideally, even implement (for example) a double price for joining. Want 10 angels - give 40K and stay without money.