I've always added Malaysian trumpet snails as they are the only snail that can protect themselves with their operculum (imagine it as if they can close a door on the opening of their shell). However cichlids are smart and eventually will get to them.
Don't worry about the gases, we just had a discussion about it and with all of my research and another well knowledgable member here we've informed people that it's nothing to worry about. If its something you're truly concerned of then just every couple of months either run a finger or an object through the sand to disturb anything that can be building up. Cichlids move a lot of the sand but there will still be areas that only the top 1/2" or so will be moved by them.
Definitely look into more rocks, holey rock is awesome but I avoid it simply because of the price. I always advise natural river rocks cause well it's free, just needs the time to be prepped for your tank (a very simple way is to scrub them real good and then let them sit for 24 in a bucket of water and bleach, then another 24 of just water, followed by another 24 hours of sitting in the open. If you can't smell bleach on it you're good.
Next thing I really recommend is painting your background. A $3 can of black spray paint will make the world of difference bringing the colors of your fish out and unlike those crappy/fake looking backgrounds you won't ever need to worry about water getting behind it.
