30" catfish for a 150? i dont think so lol, ive seen a cple older specimens over 30" also... marbled pim would eat the o.p.'s other stock eventually. I agree, a good cat to own but not in a 150....
o.p. i agree lima shovelnose would be a good canidate that could live in ur tank for life, possibly a few pictus cats also. maybe even a vulture cat... Id stick with anything maxing out at 24" or less....preferebly even 18" or less tho if u plan to house them in there for life. U might squeak by with a girafee cat but they do get big, theres a smaller species of giraffe that would be very suitable if u can find one.