I have a question, is respecting the 8x8 grid important?
Yes it is important, but don't actually punch someone if they don't respect it! A 16x16 grid is OK but 8x8 is technically the best size!
No, you can use as a personal challenge, but it's not a rule, maybe depending of how your game will be, it's important. But many games in the past jams don't have 8x8 grids. And the only rules you have to follow is the original jam rules. A good example is the games who won the gbjam3 and gbjam4, both are awesome, and they use more than 8x8px, maybe more than 16x16.
It's not that hard to respect. You should give it a crack.
16x16 is completely fine as you can just make a 16x16 tile using 4 8x8 tiles on the real hw.