I'd say no to the Leopard Plec, they reach 9 and a half inches and IMO a 20/30gal tank would not give enough space for one to thrive.
L333 is not strictly a king tiger, although very similar, it reaches close to 6" but would be ok in a long 30gal. I've found even my 3 juveniles to be VERY territorial. The king tiger is L066 and grows to less than 5" so would probably be ok in a long 20/30 gal. Again they are territorial though so provide lots of hiding places.
/www.planetcatfish.com/catelog/speci...?species_id=174
