How to alllow attackheli and its turrets to deal damage to other attackhelis in an otherwise all-PVE server? What flags or rulesets are needed? I only know to add heli and turret as their own entity groups

you would need a plugin for that.

the game sets attack heli rockets initiator as the player. so it can only evaluate rules such as players can hurt players which is not suitable. this is no different than a player shooting an AK at another player. the player is the initiator and not the AK. a plugin can get around this easily.

        private object CanEntityTakeDamage(AttackHelicopter attackHeli, HitInfo info)
        {
            if (info.Initiator is BasePlayer player && player.GetMounted() is AttackHelicopter)
            {
                return true;
            }
            return null;
        }​

Thank you!