I don't mean to add to the tension in this post or be argumentative, but not all valentini's are aggressive. My blue spot toby (basically the same thing as a valentini, other than colors) was in the tank much before my volitan and has never even come close to being aggressive with him. You just need to observe the specimen that you are buying. Does it eat aggressively? Does it chase and nip at the other fish it is housed with at the lfs? If so, this may be a good indicator of how he will be in your tank and towards your lion. All fish are different.
Perfect example: there were two posts above that said that their lawn mower's don't eat off the rocks. Well, since being put in the tank 4 weeks ago, my lawn mower has worked nicely on the growth of hair algae I was starting to get. Will he always do this? Maybe not, but he does now. All fish are different and you have to make a judgment based on your observances, past-knowledge and experience.