I suspect you haven't quite understood some basics.
ROM = like Windows. There are 'shipped' ROMs (factory/default/shipped ROMs) and custom ROMs (Dutty/Alex/.../homemade ROMs). You always have 1 ROM 'installed' on your device; otherwise your device wouldn't function (just like a PC without Windows)
CAB = applications. Ultimate Launch, PocketCM, iFonz, & iPhone interface are apps. On PC you need to execute an .exe file (mostly setup.exe) to install an applications. PocketPC's don't work with .exe installers, they work with .cab installers.
Given your posts, I think you only want to install some applications. That means installing .cab files. No need for any ****SPL here.
If you really are already thinking of replacing your shipped ROM, than you WILL NEED HardSPL. But given your explanation I don't think you were planning on doing that.
Hope this clears some things up.
Good luck and welcome...