So what does MEM /C /P say now?
I was going to suggest that the crash on boot was because your SCSI controllers have an option ROM taking up memory address space that EMM386 may be trying to allocate as upper memory. I'd be interested to see your AUTOEXEC.BAT and CONFIG.SYS now.
MEMMAKER usually does a pretty good job but there are extra tweaks you can do too. Try reading this thread Dos 6 conventional memory tricks
As for your Iomega drivers - I would try and see if there is a newer driver available that takes up less memory space. Then if you check the size of the installed drivers using MEM /C /P you can try only loading certain drivers high (using LH in AUTOEXEC.BAT or DEVICEHIGH= in CONFIG.SYS will load a driver into upper memory - to load into conventional memory remove the LH or simply use DEVICE= in their respective files). By rearranging the drivers this way you can sometimes optimise them to get the maximum amount of conventional memory.
Alternatively you can always setup a boot menu in your CONFIG.SYS to specify a normal configuration without the Iomega drivers, and one that loads then. When you actually want to use the ZIP drive, just reboot and select the configuration that loads the drivers. See here for info http://dos.rsvs.net/DOSPAGE/CONFMENU.HTM
If you are squeamish, don't prod the beach rubble.