It depends on the hack.
The ones that you've mentioned coexist. I currently have UFHack and Savory 0.6 installed.
I believe that with Savory, when you uninstall it it will revert the browser version of UFHack if UFHack as second to be installed. I believe that SS hack would also work with these two.
This does raise some issues though.
line from prettyversion.txt if of rather limited length, so hack writers should probably limit the length of the names to perhaps 2 letters and 2 digits and loose the + sign.
Definetly when you install any hacks - back up original versions of modified config files (most hacks dump them on the kindle drive) - might save you a lot of trouble in the future when official amazon update comes around.