It’s a bit of a different type of catfish, but have you considered a gulper catfish? They only get to around 12” and would do fine by themselves in a 75.
As for what’s been mentioned in the thread:
The ornate pims stay around 12”, but they do best in groups of six or more, and that size group would need a much larger tank than a 75. They are the bigger cousins of the pictus catfish and the four line catfish.
The four line catfish stay around 6-8”, but also like groups of six or more. They are the smaller cousin of the ornate pims and the bigger cousin of the pictus catfish. They will do find in a 75, but if you don’t want a group then I’d skip them.
Pictus catfish are the same as the four line catfish, but only get to about 5”.
From what I’ve read, hybrids can be hit and miss. Some of them do well while others die prematurely. If you want to go with a hybrid, I recommend you do a lot of research to make sure that you are getting a quality catfish that will be around for a while, especially since many of them are often expensive.
The “Lima shovelnose” is, as others have mentioned, actually a subspecies that stays smaller at about 12”. A true Lima shovelnose can get near 24”, if I recall correctly. However, I don’t believe any current members here have one, so that shows you how rare they are. Instead, pretty much 100% of “Lima shovelnose catfish” that are sold in the aquarium trade are the smaller subspecies that only get to 12”. Basically, if you find a “Lima shovelnose” for sale, you can confidently buy it knowing that it is the smaller variant. If it somehow turns out to be a larger type, then you will have no problem at all selling it to a member on here for a pretty good price. There is no way to tell the difference other than their full grown size. However, I believe they prefer groups of at least 3-4, so you might want to do some research to verify that before buying them.
Hope this helps and anyone please feel free to correct me if I’m wrong about any of the information I gave.
thebiggerthebetter