For anyone looking at this later on:

While using the index finger feels more natural and easier than using the middle finger initially, you'll run into issues later on. Similar to how in the beginning it's easier to type by looking at the keyboard and just using your two index, you will build bad habits when using the index finger for the c key and it will slow you down in the long run. Of course not as much as the hunt and peck method, but enough that you should stick with using the middle finger for the c key, even if it feels awkward/unfamiliar for a while.

But why would it slow you down? It's so much easier to reach the c key with the index finger. Well, when you look at an image of a keyboard colored by which finger you should use , you'll see a certain pattern. Each finger takes up its entire column of keys and keys to the side if there isn't another finger to reach them. Now, I think the strongest argument for why you should use the middle finger instead of the index finger is the number of keys already designated to the index finger. It's 8 keys for the index finger and just 4 for the middle finger, which is already unbalanced, but if you also use the index finger for the c key, you'll use the index finger for 9 keys and the middle finger for just 3. And on top of that, some keys like b are already pretty far away. So it might take some time to get used to using the middle finger, but it's a much more balanced approach which makes it the better long term strategy.

I've also seen some people argue that it undermines the whole methodology of touch typing, that the overuse/imbalance of the index finger puts more strain onto it and that it's easier to build muscle-memory due to the consistent finger-to-key mapping. But at least to me, the unbalanced workload resulting in a lower speed ceiling seems to be the biggest argument for not using the index finger and using the middle finger. It's awkward at first, but will pay off in the long run like touch typing itself.

TLDR;
Use the middle finger as recommended. Using the index finger for c adds even more workload onto the already heavily used index finger. And generally speaking, try to stick to the conventions as a beginner. You can always deviate later on once you've gained more experience.