I am prediabetic and taking 500 mg of metformin. I have lost 10 lbs and have about 30 lbs to go. Is all safe with metformin/safe with my prediabetes?

No. Alli can be taken safely by prediabetics. But it should not be taken together with metformin as it can increase the risk of a serious, potentially life-threatening condition called lactic acidosis!
For sure. Keep it up! if you hit your weight goal, you may be able to get off the metformin. The combination of metformin and Alli should help significantly with your weight loss. But be careful - the combination may cause some unwanted side effects, particularly foul smelling and/or greasy stools.
Don't u even think. About it! that can be a deadly combination, even if you are taking low doses of metformin. I you dare to try, better buy a couple dozen cheap undies. Ever seen the color of hot lava from a raging volcano? That fiery hot orange is what will stain ypur undies forever. The raging volcano will be your rectum. High risk for kidney stones and severe lacti acid accumulation, which will buy u a bed in icu.
I wouldn't. The drug you mention is designed to block fat. It will erupt, as my fellow physician reports. Mostly, this is a behavioral modification. If you eat fat, it will go straight through you. Just avoid the fat and save your money.