Pimelodus pictus can get up to 6" and are very active, most aquarist agree that a 75-90 gallon would be the smallest footprint that a P. Pictus should be kept in. A 55 gallon that is 13" front to back, that has decor/caves hardly leaves enough room for the fire mouths let alone a group of thee to six 5-6" fish.
Also when buy the fish make sure that P. Pictus is correctly labeled and not mixed up with Pimelodus blochii which can grow to 12" +. P. Blochii will have four lines down its body and a few black dots down its sides.