Yes PCLOS uses Grub legacy (thankfully) and will include other OSs in its menu.lst file at install time.
I would let PCLOS install Grub to the MBR, and if I had any further edits I would copy them from the previous OS's menu.lst into the PCLOS one.
Yes you can choose to install Grub to the boot sector of a partition instead .... I think it is under an Advanced button, but it has been a while
Using the liveCD or LiveUSB have a look through the repository with Synaptic ...... there are quite a number of lightweight WMs & DEs available.
There are also meta packages called task-<whatever> which will bring in all that is needed to have alternative WM/DEs available for log in.
Examples of DEs are
In addition there are WMs in the repo, such as
So lots of choices
Welcome aboard ..... have fun!