So saying I want to install 7 OSes in a 250GB HDD that would mean a 35GB x 7 clean split would be A-OK, correct?
That should work. It's pretty optional. If you think you may use one system more than another, make it bigger. Since you seem to be just experimenting now, your suggestion should be good and it will leave room for a swap partition.
Installing 6 Linux systems is not that unusual, any users choice. Take a look at this site, also has a lot of Grub booting tips: