multiple boot partition scheme for many OS

I've seen many guides on the internet showing Windows and Ubuntu dual boot installation, but I want to install 1 windows OS and 3 Linux distributions: win10, 2x Ubuntu, Debian.

what I've done:

I installed windows on a SATA SSD, then ubuntu 16.4. Then, I wanted to install ubuntu 19.04 but the option "Install 19 alongside 16" did not exist.

what I think I should do:

My current partition scheme is
Code:/ EXT4
/opt EXT4
/projects EXT4
/mymedia NTFSfor Ubuntu 19.04.

I am not sure if the windows installer creates its own partitioning scheme so I install it first, then resize partitions:
Code:windows NTFS // windows installation
/ EXT4 // Ubuntu 16
/ EXT4 // Ubuntu 19
/ EXT4 // Debian
unallocated // maybe some more...
/swap EXT4
/swap EXT4
/swap EXT4
/opt EXT4 // shared programs for linux
/projects EXT4 // shared files for linux
/mymedia NTFS // shared files for windows and linuxnote that I want each OS to have its own /home

also note that I install os on a SATA SSD and /projects on a faster medium, M2 SSD because I think it is better for my compilation time to have my source files there (software stores objects and caching files within project directory).

/swap, /opt and /mymedia are partitions on a HDD.

is that about right?
how do I identify the right partition on the installer?