Wow, 5 inches in 2 years for Calvus? What are you feeding them? My female is 2 years old and roughly 2 inches long. My male is around 3, 3-1/2, and 4 inches. My other male is about 2 1/2- 3 years and roughly 3 inches.
I wouldn't see a problem with a pair in a 20 long for several years if bought as juvies. Provided you add enough rock-work you could probably keep a J. Transcriptus as well. A 40 breeder would make a good breeding tank for the Calvus as they grow out down the road.