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Issues that I feel uncertain about and you may want to cover are-
If I want a PCLOS only install, what are the pros and cons of one, or 2, or 3, or more partitions, what should they be, in what order,and how to ensure they are properly labeled/mounted?
How to share a hard drive with more than one Linux distro?
Hi Lucky Blue and welcome to the forums.
To try and answer your question(s) basically, you might try searching around on google for blog posts for this type of advanced stuff. A tutorial for the type of in depth information could be a bit hit and miss given your exact requirements.
Having only one partition can be a little nerve racking for some linux users because the home directory is tied to the root directory and that means if the home directory runs out of space, there is a chance your system can be screwed up because it has a lack of space to function correctly.
However, if you are distro hopping, then having everything on one partition makes the process easy to manage provided you are careful with disk space and/or aren't worried about the risk of damaging a system.
Mounting other directories on separate partitions is normally done on servers so the data can be put on a separate device to the system data for space and security reasons. But with such big drives now and easy backup solutions, most desktop users now-a-days generally have two partitions: / (root) and ~ (home).
How to ensure they are properly labeled/mounted?
You should have faith in the PClinuxOS installer to take care of that for you.
How to share a hard drive with more than one Linux distro?
This takes some trial and error and depends heavily on the installer of the distros that will occupy the drive. Remember to backup any data you care about and keep that data well away from the computer you are working on. The main problem you can have is unexpected results of the installer of the distro that is sharing the disk with something installed previously.
I recommend having a spare drive which you can slot in and experiment with the installers of your distros. Get confident by moving the systems around from partitions to figure out what methods you like to use. A live CD can have all the tools you need to play around with partitions.
and, just for curiosity, is there an easy way to put a W partition on after installing Linux?
Not really. Windows likes to format a drive and have it all to itself. A distro's installer comes along later and creates space for linux from the unused space left on the drive. Windows isn't much into caring and sharing disk space unfortunately. It -may- be possible for you to copy/transfer a working windows partition onto a drive with linux on it. Most users would never bother tho.
At any time ask for help on the forum if you don't know how to do something. For a big job like installing multiple distos, I'd suggest posting a your intentions a few days before you are going to attempt it. Write about what you have got such as hardware/software and what you would like to do. The community members then have time to work though your strategy and avoid trying to clean up a mess.

And the result? You'll get your tutorial (as a work in progress). You get your system the way you want it. And we all get to learn something.

Enjoy your stay.