until you get a better reply
1) if you have a working gentoo assume each folder is a partition and note its data size thru KDE I use properties in its filemanager.
this is then your guide to NON HOME NON OPT folders or partitions........I tend to use opt to install software rather than home but personal choice
2) the /boot partitiion is always installed before the others unless you are lazy and have only a swap and /
3) if you will compile lots of kernels consider 100 megs.
4) have a read of quota and see if you need it if you have more than yourself on the computer.
5) you can have lots of separate /data partiitions and no-one cares if they are on different drives but if you use 2 drives it helps (me) to have 2 swaps and I have read someone who has a swap at the outside and the inside of the disk
6) you may want to consider a linux imaging program while you are still young, its helped me. www.partimage.org
and its on knoppix or sysresccd cds
7) if /var is a concern for you leave it on / as a sub-folder and make / bigger and see if that helps.
8) try reading or posting at gentoo forums I am sure they can give better advice I only answered as you mentioned grub and I try to reply to these when I can.