how much sand is too much = anything over 1" is too much. .5" is optimal
when are they sexually mature = Usually takes under 2 years. A mature female in the tank speeds the process. I have a fully mature 9" male motoro that is under a year old. He has always been with a mature female.
what are the growth rates = Depends on a few things, like diet,,tank size, water change regime. Expect a 5-6" female motoro to be 12"+ in a year... males grow slower, so expect 8"+ in a year.
would a 44" diameter rubbermaid tank/pond be big enough diameter? i would be running an fx5 on it = No, that would not be enough. Maybe for a short while, but not for life. You could ghet a year out of that, but you'll need to up the filtration. An FX5 is not enough.. they only hold under 6 litres of media, you'll need alot more than that.
what should i have in the tank as hiding places = they dont "need" any.. if you must use something, smooth drift/bog wood is best.