That person told you that, because it is easier to stock a 30gal with Tanganyika cichlids than with Malawi cichlids. If you go with Malawis, IMO you should have a one species only tank of some dwarf mbuna species. IMHO, in this case, the best choice is Pseudotropheus saulosi; because you have 2 different brightly colored fishes in one species: females are bright yellow and males are bright blue with black vertical bars.
You have more choices if going for a Tang tank. Friends here will tell you that.
In the Malawi which are the dwarfs?
Auratus (Melanochromis auratus)
Chipokee (Melanochromis chipokae)
Johanni (Melanochromis johannii) *** 8cm
Pearl of Likoma (Melanochromis joanjohnsonae)
Maingano Cichlid (Melanochromis cyaneorhabdos)
Kennyi (Pseudotropheus lombardoi)
Aurora Cichlid (Pseudotropheus aurora)
Powder Blue Cichlid (Pseudotropheus socolofi)
King Size Cichlid (Pseudotropheus sp. 'kingsizei')***8cm
Msobo Cichlid (Pseudotropheus sp. "Msobo")
Elongate Mbuna (Pseudotropheus elongatus)
Demason's Cichlid (Pseudotropheus demasoni)
Hornet (Bumblebee) Cichlid (Pseudotropheus crabro)
Saulos's Mbuna (Pseudotropheus saulosi)
Red Zebra Cichlid (Pseudotropheus estherae)
Zebra Red Top (Maylandia emmiltos)
Blue Cobalt Cichlid (Maylandia callainos)
Dogtooth Cichlid (Cynotilapia afra)
Perlmutt Cichlid (Labidochromis sp. 'perlmutt')
Electric Yellow Labido (Labidochromis caeruleus)
Freiberg's Mbuna (Labidochromis freibergi)
Fuelleborn's Mbuna (Labeotropheus fuelleborni)
These are all that I know of in my database and only 2 are under 10cm. I would like to keep them under 10cm.
That's way I think Tang might be the way to go unless you can give me more species that I am not aware of.
THANKS