StyleXP won't hurt anything. I have them both installed because occasionally I have a request for one of my logons in logonui.exe format and I use StyleXP to make sure they work in that format before I post them.
But I create my logons in .logonxp format to be used with Logon Studio and that is what I use all the time.
As far as I'm aware...StyleXP doesn't hack any files. It replaces .logonui.exe in the Windows\System32 folder but keeps a backup so you can revert to the original.
Logon Studio doesn't touch logonui.exe, it creates a file called logonuiX.exe and has Windows use that as your logon.
It "skins" you logon as opposed to StyleXP which replaces the system file, but keeps a backup that it will restore when you use the button to set the Windows logon back to default.
I suppose StyleXP could cause problems if you tried to use a bad logon file and couldn't get it set back to the normal XP logon, but going into the System32 folder and renaming the logonui.exe.old back to logonui.exe should take care of that problem.
Long winded explaination over....back to your regularly scheduled program.