If you really want your multies to be comfortable and breed like crazy, consider investing in escargot shells. They are relatively cheap on-line; I've regular gotten deals of two dozen shells for $7 or $8. Once I stopped relying on rock as shelter and switched to shells, my multies became much more comfortable and visible, their behavior changed radically (I'm assuming to something that was more natural), and I couldn't keep up with their breeding. I spent hours just watching the interactions of my multies. If you target three or four shells per fish and start with extra shells to begin with, you won't be disappointed.
Let the shell field basically be the center and front of the tank, then build up some rockwork along the back and rear corners. This will be able to house some rock-dwelling species. I've kept Neolamprologus cylindricus, N. leleupi, N. buescheri and the smaller Julidochromis species with multies with no problems other than some of the multie fry being picked off. I'd avoid the larger rock-dwelling species with multies, as the larger species can be a little boisterous and keep the multies hiding. I'd suggest not trying to get breeding pairs of your rock dwellers if you want the focus to be the multies. With the footprint of your tank, if you do go for a breeding pair, stick with one pair and one species, otherwise you'll have major confllicts.
Dudley is dead on; a 3 foot tank is a little short for even the smaller Paracyprichromis species. There are several species of schooling fish that have been captive-bred long enough and under so many different water conditions that they are extremely tolerant of different water parameters, from acidic to alkaline, soft to hard. These are my three go two species as dithers for RV cichlids, especially the last two for tangs. The first is the largest, the giant danio. They are fast, the toughest of the three and are best for mouthbrooders and larger species, as they will pick off a lot of fry from substrate spawners (including shell-dwellers). They are as big or bigger than a lot of the substrate spawning species.
The second is the buenos aires tetra. They are as fast as the danios and just as tough, and make great dithers for rock-dwelling tangs. If you house them with anything too big they will end up on the menu, but are perfect for the smaller and medium rock-dwellers. They are fast, large and tough enough to avoid any serious damage but small enough that the cichlids can defend their fry without too much work.
My last option is black skirt tetras. These are my go to for shellies. They stay in the upper levels of the tank and rarely drop to the bottom to predate on fry, they are tough and fast enough to avoid any real damage when targeted by a large male multie, and their constant movement overhead makes the shellies even more comfortable.
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