As far as layers just know that the smallest will end up underneath the largest. Sand will end up being under gravel, etc. This is known as the brazilian nut effect. lol
Air pumps and bubbles? If you like the looks of it then I'd do it.
Gas pockets in the sand? All you need to do is stir the sand when you do weekly water changes. You can use a fork, a wooden spoon, etc...I use my fingers. Malaysian trumpet snails can also do this job for you. They readily multiply and are not harmful to plants.
Black versus white...a matter of prefence. I use black as I like the way it makes my fishes color "pop" and some species prefer the darker cooler as it has a calming effect. I think white washes out the "color" of some fish but I've seen some extremely nice looking white sand tanks.
I wouldn't use silica sand...there's a good reason not to use it but that reason escapes me, atm. Someone else will chime in with the reason...
Bottom line...if it's not harmful to the fish & their environment and you like the looks of it then go for it.