I'd stay with Mojave/HFS+. Better stay off a mixed HFS+/APFS-hassle!!!
You may partition the SSD with DiskUtility. But keep in mind, that the order of partitions define, if and how you can merge partitions.
Your ElCapitan is in first place. So any other partitions come on second, third etc place.
If you install Mojave onto the 2nd partition, you cannot delete the first partition with ElCap and make it part of the second partition.
To check all functions I'd go for this partition-scheme (all HFS+) since Your SSD has a lot of space:
[1] ElCap (your current system and data) => upgrade to Patched-Mojave here
[2] DATA (optional place to migrate your data and keep the os-partion smaller)
[3] MojavePatch-TestDrive (install and check-out MojavePatch here)
[4] ElCap_Backup (Clone Backup of [1])
[5] MochavePatcher (very last partition [16GB] for system-install and -recovery). So You won't need an USB-Stick.
Make sure to have proper external backups of [1] and optional [2]/DATA before starting any further changes on partitions.
Create [5] a 16GB partion at the very end of the disk to install the patched macOS-Installer here (run the MojavePatcher on ElCap[1] and install the patched macOS installer to [5]) This partition is quite important, because You won't need any USB-Stick for future patched-macOS-Installations or repair.
Partitions [3] and [4] are optional and serve as a first playing-ground for Mojave [3] and save-haven for Your working ElCapitan-Installation [4].
After a test-drive with the patched Mojave on [3] You may upgrade ElCapitan [1] to patched Mojave. You'll see, that the patched macOS-installer on [5] comes on pretty handy for those redundant macOS installing procedures.
After having ElCap on [1] successfully upgraded to patched-Mojave You may delete the Mojave-Test-Drive [3] and add it to the DATA-Partition [2]
You may also delete an (emptied) DATA [2]-partition and add it to partition-[1].
Or You may even clear your ElCap-Backup-Partition to add it to any preceding partition ... (If you have enough space left on the drive I'd keep: [1] patched-Mojave; [2]DATA; [3]ElCap-CloneBackup; [4] patched-macOS-installer).
Have a look at the "
Early Intel - Trash or treasure? MacBook Pro 2008" thread about fine-tuning of patched Mojave.
I had four 2008/09 24"iMacs running patched Mojave as ThinClients for RDP-connections to a Win-Terminal-Server. Did run fine! After having to move to a Win10Pro-File-Server last year those iMacs got an upgrade to Win10Pro to serve as a full-client workstation. Didn't perform well under Win10Pro and heavy loads unfortunately and had to be replaced by mid2012 15" MBP9,1. Sold one of the iMacs, another one was a giveaway to a nice person, who fixed my old car. One died somehow, but one is still left for any forthcoming tasks ...
My long-time daily-driver has been an early2007 17" MBP4,1 with patched-Mojave (HFS+). Didn't wan't to let go HFS+ and 32bit-support ...
Cheers, good luck to You and the iMac!
...and upgrade RAM