elemental x kernal and pure nexus rom?

Can I use elemental x and pure nexus rom? Do I need the "Flashable permissive mode (required for some custom ROMs)" download or just the kernal download? http://elementalx.org/devices/nexus-6/

If I can not afford a new phone (even though I picked the nexus 6 for the modability) should I unlock and install custom roms?

I am a noob here so thanks for all advice! :)