I'll go over each of these and tell you if I think they have potential, and what you would need to do to flesh them out. Remember, SCP's aren't just about creating things with weird effects, it's about making interesting articles that create a good story.
-False mustache that when, worn, causes the wearer to bleed out from the eyes
Don't see much here. Just a random object with an uncorrelated effect.
-Little door that, when put on an object/animal, hollows it out, so that it forms a container.
Better than the first one. I could see this being used by some race of small creatures to create their homes. Maybe the Foundation found an abandoned village made by these creatures, and they found the doors there.
-Notepad that somehow knows about the foundation??? Files appear on its pages as they are created.
I wouldn't try this for your first SCP. Meta SCP's are harder to write, and this concept has kinda already been done by Jonathan Ball's SCP-001 Proposal.
-Warm blooded snake. Probably something more than that…
This doesn't have enough to it for me to comment on it, so I'll skip it.
-Set of two interlocking rings that cause strange mutations in nearby animals. Birds grow 6 wings, bulls grow goat and cat heads, random eyes appear on things. Kind of an angelic theme?
I'd say this one is the best out of all of them, but it will require research. Give these rings a religious background, and go full on with the angelic theme you mentioned. Do some research on those weird depictions of angels from various religions, and tie the SCP into that. It'll be a good article if you center it in real historical religions. Just make sure the effect of the SCP itself is also interesting (what would happen if it was tested on humans?). If humans took on angelic characteristics, I could see this easily becoming the center of a cult. The foundation could raid this cult and find out the rings were worshiped, said to be fallen from the heavens.
