Although it may be too late at this point; I would say that if you purchased a young red fin shark that you'll be fine. It's more of a case by case with them, I have one now and it gets along perfectly with my bala sharks, iridescent, and everything else despite being the same size or bigger. There are times during feeding that he'll chase something away but never any fin nipping. I do however, agree with the previous comment about only keeping ONE at a time.
As far as pictus cats go, they are a beautiful and very active fish but as they grow they need increasing levels of salinity to remain healthy. An adult pictus needs brackish water. Sooo my suggestion is angelica catfish. Its silver with pronounced black spots, very active, and skims the bottom almost constantly. It also has the long barbels like a pictus. If you would like to see, I even have one in my 90 gallon setup; I could post a pic.