Maybe the 55G as a semi aggressive tank with the school(s) of barbs and Rainbow Shark. The 29 a peaceful tropical set-up. I'm not particularly familiar with the upside down cat, but probably either tank, as long as they are not over stocked. The sharks CAN become aggressive, it depends on their personality, but also due to a tank being too small which a 29G is (for a young/junior to full grown fish), will increase their agitation with other fish being in their space/territory. Sharks like to have cave like areas to "hide" in. And personally I think they really like what I call a race track, and what I mean is an open space for swimming all around the perimeter of the tank (this would be at substrate/ground level), and kind of a figure 8 shape swimming space around the bottom lower 1/3 of the tank. It can have Driftwood and plants at varying heights but just so there are places to cruise around like he is on patrol of his domain! They can get large. I have a Red Tail Shark/RTS which is about 7" and a female. She is big. Fitting through small spaces is tough, and just watch out for sharp snags on DW which could hurt them when they are scared and dart off really fast. Plants are good for most fish, just check their light requirements and some like Swords like fert tabs under the substrate for best growth. And Flourish is easily used for basic water feeding of plants. Those are just my first thoughts about it.