While this may work in theory, it's completely counter-intuitive and I can almost guarantee it won't work quite right.
By running Windows 7 on Parallels on your Mac you may be able to install the Windows version of Flexi (again depending upon the version of Flexi you have) and get the program to install. However, you have to have a USB hardware key in most cases, and this device usually does not get passed along properly between the Mac OS X and the Windows OS. This means your Windows OS is not going to recognize the hardware key is even attached which means Flexi will not open.
Even if you use a software key only, there are some USB peripherals that have difficulty passing through from a host OS to a virtual OS, or won't recognize to be passed through period. This means if you plan on using the Mac computer (with Flexi running in Windows) to do any kind of printing or plotting, well, your chances of working are incredibly slim.
Then we just get down to simple efficiency. If you have to run Windows in Parallels to do your work then simply what is the point of even having the Mac? Get a Windows PC. When you run a virtual operating system within a host operating system your computer is utilizing a large amount of system resources just to run that extra OS. Depending upon the hardware capabilities of your specific mac, you may use up half or more of your available memory, processor, and disk IO simply RUNNING Windows virtual desktop, so that leaves you with very little resources left to do actual work such as running Flexi or Adobe or any other software you have open operational on either the Mac or the Windows OS. Plus you have to purchase additional licensing to run Parallels and licensing for Windows itself. This makes it much less efficient to work within Windows on a Mac computer because of this extra hardware and software overhead.
I guess it comes down to a few more details that are necessary to know whether this really will work for you. First, what mac do you have? Second, what version of Windows will you run? Third, what version of Flexi will you run? Do you plan on doing only design work on this system, or are you intending to also print or plot to devices directly from Windows and Flexi?