I dont know much but I do know a bit. I know that the HVX records all footage at 60fps even when you select say 24p. or other varying frame rates. But when you import it into FCP you have the option to basically drop the extra frames. This normally leads to larger file sizes.
As for the compressor at 720p 60. Thats not a FCP error, for some reason thats the setting you have to use. There is no 720 25p compressor. as for the varying frame rates im not sure really, I can only assume that the frames can be dropped when importing off the card.
Hope this helps somewhat