Tandanus are a tough fish. I reckon they will be o.k together as long as the catfish has a,cave or something to hide out in if needed. I kept one with GTs and lion heads years ago, he was a rescue fish from a,friend who was,keeping,him in a metal biscuit tin on top of a radiator! I was shocked when I saw a live fish,swimming in 4cms of rusty stinking water so,took him off his hands. I didn't think he would survive as he was so skinny and mistreated. After a week in clean water he perked up and,I put him in the main tank, never grew,to a large size but didn't bother the cichlids and they didn't bother him.