Heavily planted means there's even less swimming space for the fish. If you take into account graval, plants space, water level, etc, its going to feel like 6 gallons for any fish in there.
20 gallons is bare minimum, and with that size you'll most likely have to upgrade when they grow. Some websites will state some dwarfs being able to live in a 20, but that just isn't true from my experience. Once a cichlid feels "cramped," it will turn extremely aggressive and territorial, including the "peaceful" cichlids. The unfortunate part is that once a cichlid becomes this way, the change is often permanent, even if you move it to a bigger tank.
Also, many cichlids will chew up your plants...