A single female Festae would last a long time 2-3 years in a 40 before an upgrade is needed. They grow slow and usually max out at 10-11 inches. Ideally you would want a 55 at least for life though. A JD would be good too.
I have a 9" jaguar in my 40/b. I'm upgrading to a 75 here in a few months though. I think as long as the fish don't get over 7 to 8 inches the fish will be fine in a 40. Everyone says bigger bigger bigger..... it's not like all of the people on the board here has a money tree growing in the backyard.