(Don't quote me).
I'm fairly sure that this is doable, and the PC2700 RAM will "clock down" to work in the PC2100 spot, although I'd know for sure before trying it, but I believe this is possible.
Most of the time it will but there are certian RAMs that will only work in the specified speed slots. It might but then it might not. To know for sure you would have to contact the manufacturer and see what they say about it.