Q: Is there a limit to the amount of sub parented items that are then parented to main items?

A: I am not aware of any limit, but there may be one. What you have should easily be within any limit. Think of a human figure there are 30 parented parts in the fingers alone.

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As to ending up with a figure with only one material, that's because when you use the HE to assemble the props into a figure Poser puts the the 'material' blocks of code in the 'figure' section, and tells the the actors to acess the data from there by a "customMaterial 0" statement in the actor. You are useing Poser primitives and these only have one material defined "Preview". Thus all the actors are getting their data from one "material Preview" block of code in the 'figure' section. When the props were just props parented to a figure, each prop had its own "material Preview" block of code in the prop itself, and was instructed to use this internal block by a "customMaterial 32" statement in the prop.

Now this is probably all starting to sound a bit complicated, and could get more so. So lets cut to the chase, you have a lot of options but lets just look at two.

Method #1: You already have a figure, and all actors use the Preview material. In Poser, use the Grouping Tool to assign a new group with a distinct name to each actor, also assign a new material to each new group. You can then set your new materials up in Poser by assigning colors and textures etc, as you would for any figure.

Method #2: You are going to make a new figure out of primitives. This is a rather unorthodox way to make a figure, but in regards to what you are trying to achive it is probablly quite a good way, and has the advantage of being very quick and easy.

Load MinFig. Parent all the props to MinFig, or parent the base element of each parenting chain to MinFig. Save MinFig back to a pallet with a new name, let's call it "NewFig". Open NewFig.cr2 in a text editor, Search and replace all instances of the string "prop " (note the blank space) with "actor ", save the file to disk.

All the props have now been converted to actors, they are an integral part of the figuer, you can select them from the dropdown Body Parts list. In most other ways nothing much has changed, in particular in the Poser Material ditor (P4), or Material Room (P5/P6), you will still acess them as if they were still props. Because the material code blocks are still in the individual actors, and still use "customMaterial 32" you will be able to set diffrent materials for each actor. Try this as an experiment with two or three primitives to see what happens, before you use it on something more complex like your building.

For more info see this thread at Poser Pros.