It's all personal preference, the slate would look pretty cool, what you can do is try to get pvc pipes with a power head on it make it like a dodgy UGF put slits in it and just s bend it up n down your tank, turning it on every week or something just to get the circulation going if you are that worried about build up. I'm sure that the amount of build up shouldnt be that much.. have you ever thought about just using some smaller peaces and covering the bottom with a black rock pebble, it looks really hot!
but if you chose to do only gravel with little slate bits, you can still siphon the gravel and clean it. large pieces may be hard to vacuum under.
one thing you have to be cautious of is if you have bottom dwelling fish, like cats or rays, the slate tends to be sharp and as they are cruising along the bottom, they can get pretty messed up, especially if they are on the hunt or retreating from an attack.
you can use it just to stir up the water incase you really want to make sure it's clean, just turn it on when you are cleaning out your tank, and if it blows some rocks out, just neaten it up, if you are having plants in there, you need to be weary of their roots, that's why i would suggest for the UGF to blow instead of suck like the traditional ones.
http://www.sydneycichlid.com/content/?page_id=17
there are some ways where you can just have some small drift wood pieces with live plant on them, on a less than one inch gravel bottom, the plants are easy to vacuum around and can be moved easily
wow, I waffle.