No guaranty.. ammonia&nitrite is stressful or toxic stage for fishes. If adding fishes this period basically gambling the livestock risk.
Please check your ANN parameters, follow below video as guideline.. Cheers..
Not easy to pair flame angel even you get a male & female.. anyway, flame angel born hermaphroditic when juv.. they change sex when growth. Try small specimen you got high chance. as long as no aggression.
Below some learning for sharing.. In general observation. Male exhibited more black stripes/bars on body & dorsal than female.. Also, male size bigger..
Cheers and Happy Reefing..