tough to say, convicts can be nasty sometimes, but may not bother most catfish and the SD's are active enough they might be able to keep clear if they do get nasty, esp. when breeding (which convicts love to do). If I had to choose ya or nay, I would say skip them.
6 SD's might be a bit much for a 75 though. I have 4 redhooks in a 65 and it is tight.
might want to cut back on the number slightly.
they will get 4 to 6 inches + depending on which species of SD you end up getting.