Re: Can I use other stock ROMs after I have rooted and still have root?
If you install a stock rom of another device onto a Nexus 7, you will either softbrick it or it will not boot. Fortunately it's a Nexus so it will only softbrick, for non-Nexus it might be a hardbrick.
You can hope for someone to port the rom, but since LG softwares are proprietary, it will not likely happen. Also no one want bloatwares on their Nexus, so it will not happen.
For rooting a Nexus, you install it by flashing SuperSU package in custom recovery. So you can flash SuperSU package to any roms.
There are themes and launchers on the Playstore that emulate LG looks.