I think you probably have answered your own question then. I personally believe its fine to have a fish which you know will get too big aslong as you are prepared to upgrade your tank or have something in mind for the fish down the line, such a a lfs that has the facilities or another big tank somewhere. If you know that you wont be able to do the right thing for it further down the line then probably best to forget it, it can be very surprising sometimes how quick these fish can get very big. One of my local shops has a red tailed catfish/tiger shovel nose cross breed, its beautiful, very much like the fish referred to in the thread regarding the shard of clay pot that had to be removed from the fish's stomach. Its only 6 inches or so now but ive seen people say that these fish can get to 20" in the space of a year, ive seen my 3 smallest leporinus grow from an inch to 8" in a year also. Its comendable that youve done your research first to avoid any future probs.
Steve