The biggest question I have is what version of Caldera do you have? It appears as though this may be a limiting factor for you in regards to performance and the hardware OS you run. Only Caldera V11.2 180309 supports the High Sierra. Check
here for a full list of versions to OS compatibility. Based on the age of the Max Mini my assumption is that you are on a version 9.xx which isn't supported past Yosemite. There is a patch available on their site so be sure to check if it's applicable to you. There are some system stability and feature enhancements within this patch.
Second, it looks as though your hardware is well within range of all versions of Caldera, but if you running an older version of Caldera then you may not be utilizing all that hardware to its fullest. Here is a quote from their V10 promo.
The inclusion of APPE 3.3 marries this upgrade with the benefits of 64bit data handling. Users can now RIP massive files which, previously, were limited by a lack of onboard memory, leveraging their processor’s full active memory to provide additional efficiencies across the full job queue. Files with transparency and overprinting benefit particularly by processing consistently three times faster than previously in live production environments, although this increases to a blistering 50 times for some files.
As far as I can understand from the above snippet, 64 bit processing wasn't a thing for Caldera until V10. This is a huge deal, especially for simultaneous rip threads. It seems to me that your biggest gains may be from upgrading to the latest version of Caldera. That said, I do find it hard to believe that you can't get at least two printers going at the same time, and there may be some underlying issues within your setup/systems.
Like Dypinc said, you must be using a Gig switch and Gig network adapter as well as good cables. If you are using the built in WiFi of the Mac Mini then there's your problem. For sure you should have reserved addresses for the printers, or at the minimum have static addresses configured on each printer somewhere way out in the boonies of your available addresses. Like 192.168.1.201(....202...203). Just be sure that you aren't stepping on the toes of any other device given a static address such as a file server, NAS, or network office printer. If you don't know what this all means then do yourself a favor and ask your IT guy. Better yet, just ask your IT guy (or gal)
I don't know Caldera, so I cannot attest this setting feature, but if there is a RIP setting enabled called "Print while ripping" turn that off. Ripping is processor and hard-drive intense and you will be asking the computer to do too much while sending fat chunks of data to multiple printers.
Upgrade your Temp folder drive to an SSD if you can. The SSD will free up the bottleneck of a slow mechanical drive while writing those massive rip files. At the minimum I would move the temp folder to a separate internal physical drive (not virtual) to free up the read/write throughput of a single drive.
Make sure you have plenty of storage. I would make sure I had at least 100gigs of (preferably SSD) available for the temp files.
Make sure the printers are set to 280 seconds for their timeout settings under RIP TCIP menu.
Ultimately I think if you are looking for a solid system to run three or more printers that you step up and get the latest rip software, Mac OS, and possibly a personal air conditioner for that poor little Mac Mini. (If you have the Mini in the same room as those HPs then you are punishing the poor thing)
