A 40 breeder would be great for saltwater. Not very tall but pretty long and deep front to back. Not being tall is a good thing if you're considering corals, because you'll get better light penetration.
If you're considering cichlids for the big tank, the big American cichlids are really not difficult fish to keep. They tend to be very hardy and eat just about anything. They'll eat/destroy plants and dig in your substrate, so live plants aren't going to work. A sand bottom would be good. In terms of care, just keep up with the water quality. Feed them a good staple pellet food, and supplement it with frozen and live food. I set up a 10g guppy breeding tank to provide supplemental feeders to my Dempsey. Definitely not required, but I thought it would be a cool idea. Just avoid using rosy reds or goldfish as feeders.
If you do an African Rift Lake setup, it'll require a little more research because you'll have a bunch of fish and you'll want to avoid compatibility issues. Generally, it's best to choose one of the lakes (Malawi or Tanganyika) and stick with only fish from that lake. The Malawi fish tend to get bigger and more aggressive but have some amazing colors. The Tanganyika fish are more varied in body shapes and sizes. It's really a matter of preference. They're also hardy fish though, so should be just fine as long as water quality is kept high. The high pH and hardness levels aren't necessary for the survival of the Rift Lake fish, but you'll certainly see them thrive in conditions closer to that of their natural habitats.