All kinds of rocks and all kinds of sizes....from enormous boulders to gravel and sand, from rock faces to mudbanks, from shell beds to fields of vallis. It's a huge lake!
If you're after creating something like that in your 29g, then you need to mix the right environment for your particular species. A mix of large and small rounded rocks, maybe granite cobbles in one or more piles. You also need open sandy areas with shells for your multi and occies (better to limit to one shellie species perhaps?) Your leleupi and syno. petricola will want caves ideally at substrate level.
Most attempts at a Tanganyikan biotope do not mirror the blandness of the majority of the lake, it's just not that appealing. It's just algae or sponge covered boulders ...............