I have never had leleupi so I will let someone else comment on them.
Julies have been know to set up trios, but it is not the nore and forcing a trio can be disaterous. I would get 4-5 juveniles and pic out the excommunicated ones as they get run off. You get lucky and end up with a trio (almost always 1 female and 2 males), but most likely the domiant females will choose a single male and run the rest off.
I had a single leleupi in a 30 gallon long, and he terrorized the julies that were in with him - only one survived. I don't think a 20 gallon is big enough....Maybe a single pair of leleupi, or a group of julies as MalawiLover recommended.
I have read in the cooking cutter setup for 20gallon, that a pair of leleupi+a pair of Julido was fine.
I already have a pair of leleupi (bought them when there were 3cm approx. and now 1 is 1inch bigger than the other, i guess its the male right?).