For Malawaiin Species your tank Ph is a little low but these are probably domestically bred fish and they tend to adapt better than those that are wild caught. As the PH of Lake Malawi is 8.5 -8.6 PH
Here is the Water Chemistry of Lake Malawi
The size of the cichlids being one or 2 inches in that size of an aquarium is within the 1 inch of fish per gallon general rule of thumb for keeping fish however some of the species that you have mentioned are closer to adult size than others.
The Species that you have mentioned are:- Scientific name:
Nimbochromis venustus Common name:
Giraffe cichlid, Kalingo Region:
Lake Malawi - Lake-wide distribution, Deep Intermediate Habitats Size:
max 10.5 inches (26 centimetres). Males are slightly larger than females. Nimbochromis venustus (Giraffe cichlid) - AllCichlids.com - The #1 Resource for Cichlid Information Scientific name:
Labidochromis caeruleus Common name:
Electric yellows, yellow labs Region:
Lake Malawi Size:
Males up to 5 inches, females up to 4 inches Labidochromis caeruleus (electric yellows) - AllCichlids.com - The #1 Resource for Cichlid Information Scientific name:
Sciaenochromis fryeri Common name:
Electric blue cichlid, Hap ahli Region:
Lake Malawi Size:
7 inches Sciaenochromis fryeri (electric blue cichlid) - AllCichlids.com - The #1 Resource for Cichlid Information Melanochromis cyaneorhabdos Common Names: Blue Johanni
Melanochromis Maingano FishProfiles.com - Melanochromis cyaneorhabdos
The Melanochromis or (Blue Johanni as it was known in the hobby) before reclassification is actually very close to adult size which by your discription you state a few are more active which I could see these being the more agressive out of the mix in regards to the general agressive levels of these cichlids. Until the Nimbochromis venustus got bigger.
The Labidochromis species and the Electric Blues tend to be good tank mates if the tank is big enough.
With the four species all in a 30 gallon tank I would say that the more agressive ones have bullied the others into hiding. I generally do not use anything less than a 50 gallon tank for Africans and I just keep 2 species in a tank with more males to females ratio.
For the size of these Malawaiin Cichlids they can all be agressive and I do think that the agression level may have come into play with why your fish are hiding.
Talling, J.F., and I.B. Talling, 1965, The Chemical Composition of African Lake Waters
. Internationale Revue ges. Hydrobiologie 50 (3): 421-463.