You could try various search engines, including the MFK one. They are a very common fish and a lot has been reported on them. They are gregarious but there is still a dominance hierarchy within any group, which is established and enforced. 6-7 in 40 gallon doesn't sound too bad to me when they are small but when adults, they may be a bit cramped.
There'd be no debate on their size if there was no confusion with respect to their name. Pimelodus pictus and Pimelodus blochii are different fish. Since many people prefer to use only common names, they pick up the names from the LFSs, which label P. blochii, a fish that grows to 1', a striped pictus catfish, I guess to distinguish from the spotted pictus catfish aka angelica or polka-dot cafish or P. pictus. A regrettable and confusing name, IMHO.
Two-three are good numbers for fish that are indifferent to their kin. In any social group, animals establish a pecking order. In a large group, the aggression of the top animal or animals is diluted because there are many on the receiving end. In a small group, the top animal(s) let the few unfortunate ones have it constantly. That would also explain inconsistent eating. Too much stress for both dominant pictus and 10x-100x more so for the submissive / challenger pictus.
I remember your case. It always struck me as hard to understand. Perhaps can be written off as a strange exception but I think that your housing is also exceptionally large. Two exceptions are unlikely to be a mere coincidence. So there probably could be a connection.
I have 27 pictus in one 240 gal tank and 6 in another. They keep together.
