ANY Issue with xbee Communication between 8mhz and 16mhz pro mini?

I"m sending 1 pot value through serial 1 xbees to another pro mini that powers a motor. Everything already works but I need to buy another Pro-mini and was thinking about saving some Juice on one side with by using a 3.3v. Is there going to be any issues with these 2 communication with each other?
Also, could i get an explanation of the Mhz value, because all i seem to see is that "it's faster". An example of why and when you'd want to use one over the other would be best.
Thanks in advance guys.