I have been having external runtimes for a couple of years now in P6 and also since I got P7 a few weeks ago and you can name them what you like so long as it starts with "runtime"  My runtimes are labeled, for instance, "runtime Aiko" and "runtime hair"....I usually never have issues with poser not finding the correct files except for one particular hair model but I attribute it to just another bug in P7
Hope this helps.