If you mix substrate sizes, over time things settle and you'll have the small diameter substrate sink below the larger grained substrate.
The reason people complain about cleaning sand is because you can't use a gravel vac on it - there's nothing you can get near the sand with a syphon with that will pick up poop and not pick up sand grains when you're not careful. You can use a hose and syphon out the poop sitting on the top without the sand if you're careful. If you do syphon out sand ... sand is CHEAP compared to gravel (even the PFS sand that is now close to double what it cost 10 years ago.
The other reason people complain about sand is because the poop is normally a larger particle and sits atop the sand. If you had gravel, it'd sink into it. Then people would compare the sand tank verses the gravel one and say "wow, there's a lot of crap in that tank with the sand, unlike the tank with the gravel". Thing is, that poop sits and decomposes in the gravel tank. In the sand tank, you can syphon it out.
OR, if you have sufficient current, the poop in the sand tank will at some point get picked up, swirl around a bit, and make it into the filter where it's easily removed. The larger the grain sand, the greater the current you can have to swirl up the poop and not make a mess of the sand. Thus, PFS (pool filter sand) is the often recommended sand.
The special "cichld" gravels typically have limestone or some other passive buffer agent to help maintain a higher pH. This is cool, but you could just as easily add some limestone, crushed coral, etc, to the filter to help buffer the water. The buffering is greatest where you have greater water flow over the buffer. In the filter is great. As a substrate, the stuff on the very top gets good flow ... the lower depths of the substrate gets little flow unless you have a UGF ... and we don't recommend a UGF to ever be used for cichlids.
