rv415s05 In the meantime, it explain why my boot menu have only two os(KaOs and Solus) after successful installation.
Solus uses systemd-boot too. Now, I have to boot Linuxmint by F12 boot menu.
Is there any way to fix this? I mean let Linuxmint be on KaOS boot menu. I installed rEFInd in Linuxmint, it didn't work.
Systemd-boot can only pick up OS's in the same $esp. So if Linux-mint $esp is not the same as KaOS and Solus, it won't be picked up. The same applies for rEFInd. But it (refind) can detect efi files in other $esp's and boot them up. But that means it will boot through their own efi files and that usually means through their grubs. Grub 2 can detect other OS's not in their own $esp and boot them.
There can be ways to boot other OS's not in the same $esp in systemd-boot. That involves copying over their vmlinuz and initrd files to systemd-boot. You have to copy as sym-links won't work on fat32 format partitions.
Another way is to copy over the core.efi files and the boot will 'chainload' to that efi file effectively going through their grub (usually).
I would advise against using Ubuntu (and derivatives) in systemd-boot as their kernel generation makes a sym-link (vmlinuz) to root directory. As said, sym-links won't work. Also if using same name vmlinuz and intrd files, like arch and kaos, (both using vmlinuz-linux and initramfs-linux.img) or like Ubuntu and Linux Mint, have them in the same $esp (/boot), systemd-boot would be perilous (same as in rEFInd). But grub 2 will present no such problem for any multitude of OS's.