What now? How can I make the whole drive dedicated to PCLOS (preferably just one big partition rather than two or more)? If I'm only running PCLOS do I need the two small "Grub" partitions at all?
None of the partitions are for grub.
One will be for the Operating System (the root partition with mount point / )
Another will be for the user settings and such things as changes to the Desktop look and will have the mount point /home )
The third partition will be Swap .... space your system can use if it ever runs low on memory, and also the place that the system details are stored when the system is suspended to disk.
It is advised to continue to keep those partitions separate to everything else.
Now for the space released by deleting the Windows files from that partition ..... I suggest it is best to keep that also on a separate partition ..... lets refer to it as your Data partition.
So if you reformat that partition for use by Linux only -- say as ext4 type format -- it can be used by you to store all the files of every type you wish to keep safe.
I suspect that most of us have four or more partitions just as described above.
In fact I have several 'data' partitions, used to store different types of files ..... such as Music on one, Videos, ISO of CDs etc on another and VBox disks on another.
Through it all, regardless how many 'data' partitions I have, I continue to retain the three main partitions .....
/ .... for the Operating System
/home ... for all the user files for as many users as are created
swap ..... for suspending .... its size is approx 2 x the size of my RAM