my friends brother says that you can blow a transmission by down shifting, is this true? i got a 93 f150 ext. 4.9L I6 5 spd with a mazda transmission(stock).

Brakes are much cheaper to repair so it would be best to use them. It depends on how much you downshift and how hard. Clutches are also a lot more to replace than brakes. Jack Rabbit starts and stops also cost you much more for fuel.
Your paying the bill so you decide what to do.

Any transmission can be damaged if you dont know how to shift one, whether you`re upshifting OR downshifting. But you`d be hard pressed to blow one up from properly doing either one, that is unless you`re doing something like trying to downshift all the way to first gear from 80mph. Transmissions are designed to be shifted either way, the synchronizers job is to make the shift as smooth as possible without grinding. You can actually upshift or downshift without using the clutch if you get the rpm right.(not recommended, just saying you can) When you downshift(using the clutch) just bring the rpm up a little and when you get the rpm right it will shift without effort. Downshifting is necessary sometimes and beneficial under some driving conditions, like driving in the mountains. Downshift and use engine braking going downhill instead of riding your brakes and getting them hot where they may fade when you really need em. You wont hurt the engine or transmission.