You might want to keep in mind that otos can happily nibble away at the near invisible algae that pretty much every tank has. Nor are they large, voracious fish that require huge amounts of food.
If they have nicely rounded bellies, good color and occasionally zip around the tank you're likely fine. Offering a variety of food helps ensure their health--and your other stock will enjoy it anyway--but I wouldn't stress. In terms of how to feed, I slice off small pieces of whatever food I'm feeding---zuch, peas, asaparus, green beans, kale, spinach, radish or turnip leaves, brussel sprouts, etc--blanch it so it'll sink and just drop it into the darker corners of the tank or clip it to a lower piece of driftwood in a shaded area; otos tend to be shy feeders so a huge slab of food clipped in a brightly lit area isn't going to encourage them to try it out.