Diatoms will go away on there own eventually...no need to get fish for them. If you are starting to get green algae I expect your overstocked tank has to do with it. As HN already mentioned more fish, more poop, more nutrients...more algae. CAE will outgrow your tank, become too aggressive when older and otherwise just become a big waste machine. Not to mention the otos will have to compete with the tiger plec for food, not something they are good at. Are you supplemental feeding? What is your maintenance routine like?