By convention, compound SI units (e.g. N•m) are separated by a space or multiplication dot and are not just smashed together. The lack of such a separator in the above example implies the "milli" prefix.
No one uses millihertz and neither meterhertz. If you want to do it correctly the first one would be acceptable for milli as scale. The second one would need to be written as m • Hz.
All metric unit prefixes are possible. Like femtohertz, picohertz, centihertz, decihertz, decahertz, hectohertz, petahertz, zettahertz. Not sure if they are used often.
Well Hz is a frequency per second. It doesn't make much sense to use them for low frequency things but it's still possible. Just most things that happen less than once per second are usually described in units like seconds, hours and more.
I think there's something like 43/500ths of a Bigole Hertz to a regular Hertz; but that's if you're using English Bigoles. I think it's 43.9/500ths for a US Bigole.