8gb will not be big enough for the iso files, i tried to get W7, Vista and XP onto a 16GB.
Easily, just partition it like a hard drive and look at what distros offer USB booting. Should be able to add a GRUB menu on boot.
Oh, you want Windows? Move along.
I don't see how that helps at all, you have no idea why he wants to do what he said, it may very well not be for the same system anyway.
In which case yeah, just partition it and put the bootable USB image on each of the partitions. If you actually wanted to install them then you'd have to look for specific distros which support that i believe.