There are 3 key differences between KOTOR Nihilus and the one in GoH.
- Mask has gold detail instead of silver
- Padded or layered gauntlets over the gloves
- Added hip fabric that flows separately from his normal robes
The 4th potential difference that is currently being debated is if the front panel of fabric is a separate piece that is over the robes (like a tabard), or part of the robes as normal and the game model is just wonky.
Now if these differences mean having a totally new costume for Nihilus, then go for it. Or, it can be a variant on the already existing KOTOR CRL. I personally could see this being a whole new CRL due to the differences, but that is just my 2 cents. It comes down to if someone likes the costume, then make it! Have fun and do a cool character.