You can boot TWRP,
fastboot boot recovery(TWRP).img
You can do what you need to, and if you decide to install go in the menu and install but is not mandatory. You can go back on the first page for the exact instructions. And yes, if you flash a ROM you would be on another slot with whatever came with the rom but I believe there is an option now for being able to keep TWRP but maybe someone else knows a little more than me since I haven't got to really flash any ROMs yet personally.