I agree with the notion that the larger the tank, the lesser the potential for any trouble.IMHO Unless the L Marmoratus and the L Pictus are in very large aquarium like Viktors 4500 Gallon the odds are there is going to be trouble. Even if the fish are of similar size there is high potential for fighting. With the size differential you are considering I think the odds are even higher that trouble will ensue. Anything from fin or body damage all the way to death by overt aggression or swallowing of the smaller fish in this case the L Pictus. Of course there are exceptions to the rule.
Jeff V victor448 pointed out and I agree that this is L. marmoratus, not pictus. Aquascape made a mistake.Aquascape has 2"-ers for $30 in stock their site says http://www.aquascapeonline.com/prodView.asp?idproduct=489&item=Marble Sailfin Catfish 2