Shovelnose are still capable of potentially reaching 20-36 inches(eventually)I read that Shovelnose were smaller then Sterlets that is why I was thinking about one, if I can't find a source for them I could always get a Sterlet and make the pond bigger. As for tankmates I would definitely do what you said before thinking about adding anything else. Catfish do not seem like a good idea based on what you said since they are such aggressive eaters.