I'm sorry, but I totally disagree with putting festivums in a 30 - 40 gallon for a couple reasons.
First and foremost, festivums grow bigger than you may realize. My males were 8"+ when I got rid of them. Yes, females stay considerably smaller in the 4 - 6" range, but you can't guarantee the sex of your fish at the store and so you don't know which you'll end up with. At this size they were active and fairly aggressive, so even in a 210 gallon tank they caused a lot of commotion and were constantly chasing and nipping other fish. A lot of people here claim festivums are peaceful, but they are still cichlids -- as they grow larger and get older, they tend to be more pushy.
Second, festivums are a shoaling fish. They do best in groups of six or more. You can sometimes get away with four, but to spread out the aggression it's best to have six. You can keep a single festivum if you want, but then you miss out on their interactions within a group, which is what makes festivums interesting in my opinion.
You've made a ton of posts here, asking if you can put medium to large cichlids in fairly small tanks. Maybe you should instead consider looking into cichlids that will fit the tanks that you currently have. There are lots of interesting cichlids that stay 6" and under.