MicroRCracer,
I don't mean to put a damper on your creativity or anything, but the simple answer is NO!!!!!
The absolute rule when adding a cell to an already existing pack:
Try to use one as close to the originals as possible.
What is meant by that is the brand of the cell and mah. Everything else HAS to be match EXACTLY!!! This would be stuff like:
size of the cell-------ex. AAA, sub C, 1/3 AA
type of cell-----------ex. NiCd, NiMH
voltage---------------ex. 1.2, 1.5
So in answer to your last question, NO NO NO, please don't try mixing NiCd cells and NiMh's. They are totally different technologies which are NOT compatible at all!!! They have to be cared for and charged differently.
If this was done, doesn't matter which type was in the majority, the pack would NEVER work properly if it worked at all!!! Meaning, if you did manage to get it charged it would NOT last very long at all and with in 2 or 3 charges the pack would be totally dead if it hadn't yet exploded!!
IF I scared you I mean to!!!
Please, please do NOT try this EVER in any situation RC or unrelated.