Thanks for clarifying! I'm glad I did it right on the licensing side then. I've never done it before so I was worried I would have something wrong on my end.
That makes a lot of sense with the indexed mode. I'm not sure if it would work on other GameBoy engines but it does work specifically in GB Studio which can build GB-compatible ROMs. I made the sprites the same way I did for my GB game last year and I did run that game on both an original GameBoy and a GameBoy Advance. So hopefully these sprites should also work. I didn't compile the sample scene into a ROM to test it but it was working in-engine.