If you can't upgrade then you won't be able to keep either species for life. TSN's will outgrow it very quickly, usually reaching at least a foot by the time they're 1 year old. I have one that is only 9 months old and is already about 13-14 inches. At that size they're already too big for your tank. I'm less familiar with Zebra Shovelnoses but I do know that they will reach about 18", which is also too big for your tank. I would recommend not getting one of these species with the hope of getting rid of it when it get's too big. It's harder than you think to get rid of large fish, so it's never a safe plan.
You should check out planetcatfish.com. They have a search feature where you can narrow down your results by maximum size of the fish. With a 24" wide tank you should be looking for a catfish that get's 12" at the absolute maximum, but even that is pushing it.
Also keep in mind that most catfish grow fast, and clown loaches grow very slow. Considering most catfish will eat whatever will fit into their mouth, the loaches will probably end up being eaten, which can actually be dangerous for the catfish because CL's have these little spikes near their mouth that can cut up a predator's insides.