When I had separate runtimes Poser could not find the geometries or textures one time out of ten even with absolute paths and paths checked by 3rd party utilities... for a while I ran separate runtimes with all the textures and geometries in the main runtime but that made managing files for moving, copying, and deleting a pain. Now I have everything in deep sub-folders in my main runtime but Poser parked on my empty "download runtime" so it opens quickly... I uses Dimension3D's system enhansement that seamlessly loads any Poser content found in any Windows or Explorer window into Poser with the double click... it also gives all these content files icon based on the Poser .png or .rsr. thumbnail. It also saves into any OS window and does tricks like "load conformed with morphs and deformers". I usually use the freeware 2xExplorer instaed of the satandard Windows explorer because it has two view ports, location bookmarks, and an incredibly fast search.

That all said... yes... just redirect the new content into the new runtimes if you do choose to do it that way. I had runtimes for each figure and their clothes, runtimes for types of scenes and props and a runtime for shared resources such as hair and jewelry. Something to consider is creating a base runtime with your main figures and main content and installing content on a project by project basis into a "working" runtime which can be deleted with each project at hand. This worked for me when I was using Poser for commercial work and kept close tabs on the resources used in each project for legal reasons. I would then use an extractor to archive my .pz3 with the referenced content. If however you are doing this all for fun using/deleting runtimes will give you terrible grief if you want to be able to open old .pz3 and be certain of finding all the referenced content.

Honestly, I have used Poser with huge runtimes even before switching to a library replacement and not had the start up problems you describe. I suspect the problem is your software install being slightly corrupted and not because of the big runtime.

BTW Dimension3D has a free library alternative loader script that opens up a standard Windows window for you to load the content from... you should try it... their are about half a dozen Library alternatives available, most free, and all superior to the Poser library pallet in my opinion.... the Poser treatment of the loading from the library is deeply flawed... I find the Dimension3D tool loads content much faster and with no failure rate at all. I found the free alternatives slower aside from ockham's loader which had no thumbnailing but was fast and reliable and the new free script from Dimension3D which is fast but doesn't have the benefit of working partnered with Windows Explorer or 2xExplorer as does his commercial script.

I have no advise for you if you are using a Mac :(