The Turtles are all custom sprites not directly based on any previously existing sprites. I did model their attacks off of frames from the TMNT Tournament Fighter game and TMNT IV, both on the SNES, but since those sprites are easily 4 times larger than the ones in this series, it was simply as a reference and for ideas instead of tracing.
However, when we talk about tracing and pallet-hacking, Shredder, Tokka, and Razar are all direct sprite hacks. Shredder is a redo of Magus from "Chrono Trigger". They're both bad asses, so I didn't feel bad about it. Shredder's helmet, though, was a custom work.
Razar is one of the wolves from "Secret of Mana" while Tokka was based on the Reptites from "Chrono Trigger".
Most of the sprite work and hacks for this series were done well before the arc. I wanted to make sure I had everything ready ahead of time, as otherwise I'd have never gotten the project done. Having to do sprite work (and make it look good) is a time consuming process. To note, Tokka doesn't do anything much in this series because I only had two frame ready of him by the time I started in, and I wasn't going back to make more while I was in the midst of it.
And, yes, that is the classic line of Tokka and Razar's from the second TMNT movie. It's how you know it's them.

When evil spreads across the land, and darkness rises and the monsters roam. When the creatures of the night make beautiful music, and the things that go bump in the night go bump with greater enthusiasm. When the world is in peril and is in need of a hero...

These guys are, sadly, the best the world can hope for. These are the adventures of the heroes of CVRPG. They mean well, they try hard, and occasionally they do the impossible...

They actually do something heroic.