Normally grub2 should autodetect all OS (with typical scripts as packaged by debian and ubuntu at least), when update-grub is run. A new installation should run this by default, so probably your grub is the one from the latest installation and provides a menu with all detected OS. You can have a look at the os detection scripts in /etc/grub.d/.
Swap space can be shared by all linux installations in a multi-boot system and will probably be automatically detected during the installation.
Note that the grub installation is probably the one from the last installed linux. The files like grub.cfg will be updated on all systems separately, but the grub in the MBR/bios_boot_partition points only to one of the systems. This means you will probably need to update the grub menu in the last system when you install a new kernel in another system. Maybe you can install grub of one system in the partition instead of the mbr and the other system detects this and adds an entry "boot grub from sda3", which loads the other grub (giving a new menu) but I did not test such setups.
