I want to try to make an overlay, but I won't be drawing it from scratch, I'll be drawing over the base (assuming that's legal)

there's only a few poses that look good for it, but if I use them the fact that they're that color might be completely covered up. Like I am thinking of using the bloodred or reborn pose for the overlay. The blood , or fire, will be completely removed and covered up when I'm finished. Do I still need to turn the pet into a Reborn evne though it will look NOTHING like a reborn when I'm done other than the pose?

Basically, I am making a fan pet, and it's going to end up like this:

so pretty much everything that makes it a reborn other than its pose will be changed

Personally, I would think you should change it to reborn (or Bloodred, if that's the pose you end up using).

BUT, you really need to submit a ticket for this because its rules interpretation and we aren't the ones judging if your overlay is within the rules or not.

even if you use a pet of a certain color as the base, as long as the final product resembles the actual color/species combo, it's fine. so for instance if i wanted to make a custom overlay for a glacier sheeta, i could edit a reborn sheeta to look like it's made of ice. there is precedent for these sorts of edits, but if you wanted to be super extra sure about your particular case you could file a ticket.

I did the same, used the Nightmare Serpenth to make a blue and white overlay, submitted it in a Dusk Serpenth and it was approved!

As long as the overlay isn't misrepresenting what a pet is, this shouldn't be an issue. Like if you made a reborn ruffie overlay and the pet was a common sheeta I could see it being problematic.

Like I drew over the Dark Matter Lain for my pet

I mean it's a custom overlay you're paying to upload, it should be up to you what official pet best represents what you're going for.