I would recommend Direct drive preferably and please forgive me, I misread your original post (I was a bit sleepy as I am now!), 11.1v with the brushless you have will be fine as long as it doesnt spin too fast (say somwhere around 30,000 Rpm), the mamba will have far too high RPMs at 11.1v, you may well be advised to use a plane gearbox if it is much above 3200Kv I'm not sure yet that my mono setup will be controllable at full throttle, watch my Gecko thread and I'll let you know how it turns out, it may help, also try Sonnich's Hydro Sprint thread.

If worst comes to worst you can buy a slower micro brushless and use the ESC you have, they are pretty cheap on Ebay for a generic Feigao.

Finally start with a small prop and move up following tests, at high rpm's like these you will need to stay reasonably small