There is no cheat I am aware of but here is one way to do it... Get 5 Meowths and 1 Mewtwo Level 80+ and teach it Thunderbolt, Psychic, Ice Beam, Shadow Ball and walk around killing random trainers. The Meowths "Pickup" ability will give you random items, including rare candys.
You get it from the action replay
All pokemon's highest level is 100.
You can only get that many rare candies by using an Action Replay.
train it to level 100!
None unless you use Gameshark
You can Give it to your Pokemon to hold not use then Clone the Pokemon at your PC
In Pokemon firered you can't, but in Pokemon leafgreen you can get it then trade it to Pokemon firered.
cloning glitches
100 steps
The max number of most items and healing items is 999 for D/P/Plat.
There is no Pokemon convention in firered.
It's easy all you have to do is trade with leaf green