I have seven Threadfin Rainbows in a 75g tank and you are right their throats are small.
I feed mine thawed frozen cyclops, daphinia, rotifers,
brine shrimp plus specially crushed flakes. If you can get ahold of some live MicroWorms they would love them.
I noticed you said "a" threadfin. They really do better in groups but since your tank is desperately overstocked I personally wouldn't add anymore.
I just noticed your
ammonia is at 3.0. That is a toxic level, I recommend you do a 50%
water change today and another each day until it goes down. At that level you may start losing fish.
carol