I started with Poser Artist ( Poser 4) and was happy to get Poser 6 when it was launched.  However I moved to Poser 7 because it performs more efficiently. 

There are no issues with using runtimes- including external runtimes in Poser 6 or 7 - the main limitations in my experience are the python scripts.

Python scripts need to go into the base runtime, and scripts which were wrttien for Poser 6 do not always work in Poser 7 ( some have been modified by their authors, but you will need to check carefully).

Eternal Hobbyist