Peter is right. The fish that were formerly known as Meridionalis are moved to the Rhabdotus complex. The smaller fish from the Rhabdotus complex are now labeled as (ex)Meridionalis. There isn't much diference between the Rhabdotus and (ex)Meridionalis except for the size and some small color differences. Check
www.aqvaterra.com for the several species.
As for your situation,....they need the cool period of at least 3 months and tolerate temps during summer up to 24C.
Short term they can handle higher temps. I expect that the Laetacara dorsigera will be fine on 24C so that won't be a problem. Fish from the Rhabdotus complex are not aggressive but they do become more territorial at higher temps. For the fish a higher temp is associated with summer and time to reproduce and spawn. All fish from the Rhabdotus complex should be kept in small groups of 5 or more. They are no harem builders. The 40 gallon would be OK for 5 (ex)meridionalis but this would full stock level imo and no more room left for the Laetacara dorsigera.
Some real experts on CRC can give you more info. Bas Pels is very knowledgeable and Felipe Cantera is one of the 2 people in Uruguay that is allowed of exporting fish.