I just tried this, and I can assign up to 6342 MB memory to the VM in Hyper-V, with 8192 MB assigned to the Windows 2012 VM in Fusion. I suggest double-checking the amount of memory that you assigned to your VM in Fusion, and making sure that Windows actually reports the correct amount of system memory.
Also, make sure to set the Fusion VM's guest OS to "Hyper-V" instead of "Windows 2012" if you are planning to run Hyper-V nested in a Fusion VM.
As far as running OS X VMs in Hyper-V within Fusion, it's probably legal (since the OS X EULA specifies that the physical hardware must be a Mac, which it is), but I have no idea whether Hyper-V would actually support OS X as a guest OS. A nested VM is also going to be a lot slower than just running a second VM in Fusion. Is there a reason you can't just do that?