The numbers after /dev/sda will change depending on how you partition your SSD, as well as their order, and I’m assuming that your username will be “macaroni” for simplicity, but look at the idea. (Also, you don’t need to mount your personal files partition in /home/macaroni/storage. It could be mounted anywhere you want, even /randomjunk/catpix/dogpix/mypartition.)
If I recall correctly you don’t need to mount the partition with grub (the bootloader), but do check it with other users as I’m not sure of that. Once you update grub in that distribution, it’ll automatically detect “look! There are other systems here! I’ll add them to the boot options!”.
Also, someone mentioned creating the partition first. how would that work out if you’re still running a distro?
You should only create, delete or modify partitions of your SSD from a live USB. Never do it while the system installed in your SSD is running. Those partitions will stay even after you reboot.
I recommend creating the partitions first, then installing your distros. This way you’ll have better control on how to organise your partitions. For example, if you decide to install Arch on the third partition, you can simply say it “hey, you shall be installed in /dev/sda3”, no matter the order that you’re installing Arch vs. other distros.