Vensim Professional and DSS will install to a program named vensim.exe. Thus if you upgrade from Vensim Professional to Vensim DSS the installation will simply replace Vensim Professional. Vensim PLE and PLE plus, on the other hand, install to different names. Vensim PLE installs as an executable named venple.exe (Vensim PLE on the Macintosh) and Vensim PLE Plus as an executable named venplep.exe (Vensim PLE Plus on the Macintosh). Therefore, you can install Vensim PLE or PLE Plus and another Vensim configuration in the same directory without any conflicts. The support files installed for all configurations are identical, though some configurations install additional files not needed by other configurations. Vensim also stores a limited amount of configuration in the files vensim.ini (Professional and DSS), venple.ini (PLE) and venplep.ini (PLE Plus).
The long and short of this is that installing Vensim PLE along with another Vensim configuration in the same place will not cause any problems.