Can I install VMWare Workstation in a Mac's Boot Camp partition that has Windows 7 installed in it? Is there any technical restrictions? Will there be a problem if I load the Boot Camp partition in VMWare Fusion, after VMWare Workstation is installed?
When in Boot Camp, you are running Windows 'on the metal', as in really no difference than Windows on any other PC, except on Mac hardware with some Apple drivers for that hardware (like fan control, iSight, etc).
With that in mind, there would be no issue with installing VMWare Workstation in Windows while in Boot Camp.
As for what happens when you launch the Boot Camp partition in VMWare Fusion, I have not tested that, and not entirely sure how it will work. I think that it will launch, and possibly even work (except for being rather slow, because you would start virtualizing while virtualizing). The only limitation I can think of is that if the hypvervisor has a way of knowing it is currently already using a VM, and therefore not able to virtualize again.
One advantage of using Fusion on the Mac though would be that they can use the same .vmdk files. Therefore, you theoretically could put the disk images on a shared disk (for example on the Boot Camp Partition itself, since the Mac can easily read it), and then launch the vmdk no matter which host you were using (Mac or Windows).