It's been awhile. Anyway, to answer the questions:

No, none of the polycounts are too low for Poser.

But, there's really no such thing as "too low of a polycount" as long as there's one poly. (There's "null objects", though, but that's not the point.)

What you have to think of when dealing with polycount and topology is "What do I want this model to do when rendered and how versatile does it need to be."

First off - The working scene window is rendered using OpenGL. So, when you're modeling something, you may wish to consider how it looks to the user in the OpenGl preview window. What does OpenGL care about? For one, probably the most noticeable, it cares about surface normals. But, specifically, it cares about surface normals as they are determined by "vertex winding order." Most 3D applications will show the effects of surface normals, either directly or as a toggled display option. Other issues like transparencies, shading issues with certain tightly packed vertices with not enough reference points, like "long, thin, tris" or with n-gons, both which cause issues in Firefly and, I would assume, some in Superfly, might be considered as well.

However, in your case, dealing with sharp edges and inorganic geometry, the biggest consideration is "smooth shading effects" and how topology and density effects those when the object is rendered.

So, do you intend to have "Smooth Shading" turned on for your model? Yes or no? THAT is the most important question when it comes down to modeling inorganic, sharp, edges in Poser.

If the answer is "Yes", likely due to a model that may have both sharp edges and some inorganic smooth, curvy ones, then mesh density is something of a priority, but only for specific areas. This is where the subject of "beveling" comes into play.

But, let's say you think that you can easily manipulate that using the Smooth Shading controls in the Object menu and still get away with very low polycount, without beveling, or even a minimum of beveling. OK, fine - Try it and see. One issue you may encounter with something like a single-polygon wall is that while the edges don't appear very distorted with very low smooth shading settings (like .20 or something) is that, along its length, the wall "warps" outwards a tiny bit. Why? Because the engine doesn't have enough info given to it by the vertice count to decide to keep that "flat" plane really flat when rendered.

In short: A one polygon wall, without any smooth shading effects turned on for it in the Object menu, is going to be fine if you want nothing more than a large, flat, surface. If you need to have Smooth Shading effects for it or desire that option to be available, to limit the amount of work the user has to do when pressing the "Do Art" button, then that wall will need more polys and, perhaps, it may need beveling if it is anything other than one simple plane. (Doorways, windows, physical joining with other surfaces, all will most certainly require more polys than "the minimum" to avoid smooth shading issues if Smooth Shading is turned on for the object. Even then, there are times when such features will confuse the renderer despite Smooth Shading being turned off, so properly placing more polys to give the renderer the information it needs will be in order.)

The majority of your instictive decision-making process when modeling for Poser will be guided by your practical experience. With so many variables across models that may even look very similar, you'll have to learn through experience what to do and what not to do.

"Planning" is the most important step towards solving any problems before they occur, so think out your model, first, and decide what uses you want it to be suitable for. Only then will you be able to know what the polycount needs to be in order to avoid problems. There is no hard-and-fast rule involving density or polycount that is applicable across all models.