what kind of decor to use is really a personal choice, go through the media section and see if there's anything catches your eyes. if you worry about they kick up sand then use small gravel but if they want to they could kick up those gravel too. if you go with pool filter sand they sink down pretty fast compare to play sand so you shouldn't have issue with filter sucking up sand. and unless you have HOB filter then you shouldn't really worry about it as filter pad will filter out the sand before they get into return pump in canister/wet/dry filters.
if you get them at 9" then 3 would be it, personally I wouldn't add anymore unless you are willing to upgrade.
if you get them at 9" then 3 would be it, personally I wouldn't add anymore unless you are willing to upgrade.