Short Ed25519 keys: can I use as ID?
What's the odds of system generating same user ID for more than one users?


You need to have way more than $2^{100}$ key-pairs / users for a collision.

With Ed25519, the public key is $P=[a]G$ with $a$ being the private key and $G$ being the public generator point of the curve. Now because of the determinism of the scalar multiplication, if $P=P'$ then $a=a'$ and thus the users share the same private key. If both users are using a good random number generator, they never pick the same private key. More precisely: If you generate less than $2^{126}$ key pairs there shouldn't be any two key pairs with the same private key, because you would be looking for colliding random 256-bit strings.