I have 6 threadfins in a 55 gallon with 10 glo light tetras, 3 praecox rainbows, 5 mollies and a few others. In my tank they stay in the top third and eat like crazy. The main thing is to crush the flakes or make sure that any frozen food is shaved a bit to give small bits of food. The smaller bits tend to float around more and gives them a chance to eat. I usually crush the first bit and then put in some whole peices. The larger fish tend to go for the larger flakes and the threadfins get a chance to eat.
It depends on what size they are. I started keeping my jack dempsey with otto cats since its about 1 inch, now its about 6 inch and still get alone fine with otto cats. I do a head count once a week, none of the ottos were eaten. Start them small.