As long as there is a ROM for it you should be able to run any OS you want. You may want to check for the ROM you want before digging in. I would look over at xda-developers.com for a ROM and directions or help rooting.
What are you needing to do that Kitkat doesn't allow? After you root, you can find a patch or fix for just about anything. Patches and fixes are easier to use than installing a whole rom. Many are available in the xposed framework app.
Try a third party launcher. I believe Nova and Go launcher both allow you to hide certain apps.
In order to install a rom, you'll have to have a custom recovery and that isn't available for GS5... Yet. Plus I don't think you can go back to jelly bean once you are on Kitkat. It has to do with the bootloader being incompatible, I think.
If you have root access an app called hide it pro can hide apps lock them and hide many other things it also can be displayed as a audio management app. Although as previously stated many launchers can hide apps apex is the one I'm using and it has it for free no need for root at all