Can I use the 'i' variable in different mc's in one swf file or do I have to use another letter e.g. 'j' in the other mc?
You can use the i variable mutliple times. For the most part, just avoid using it multiple times in the same function or frame of code.
any variable (like i) used in different movieclips are different variables. ie, they have different scopes.
so, if you define i in one movieclip, it will be undefined in all other movieclips.
if you re-use i in the same movieclip, there are no problems as long as you don't need to retrieve a previous value of i that was subsequently changed. ie, there's no problem with:
//do whatever else
all in the same movieclip.