Flashing a 'generic' TWRP Long story short, my phone (Pantech IM-100) does not have a device-specific TWRP and I wish to root with SuperSU. 

Do I risk bricking my phone if I fastboot just any TWRP version? My impression was that the fastboot command differs from flash, hence it does not overwrite stock recovery and is only temporary.

Thanks.
It is highly recommended not to do so. The TWRP is a touch-based recovery, meaning that it needs to initialize touch screen (actually every related hardware) before showing the main menu. If you insist on flashing a 'generic' TWRP to your specific, non-matching device, you will end up getting stuck and your recovery will never boot until you flash another compatible one.
The same applies to CWM as it's button-based. Given that different devices often have different volume keys &amp; power key mapping &amp; signals, some devices can share their CWMs but not all of them.
You had better contact the TWRP dev team or do some searches in the XDA forum, with the hope that someone might have already adopted TWRP to your device.  

Edit 1: Thanks to @AndyYan who pointed out a mistake in this post. Corrected now.