Can I add a second type of the same projectile to exist separately?

Yes they can, inheritance only re-uses properties of an already existing actor, it doesn't affect it unless you want it to.

You're probably looking for this:

https://zdoom.org/wiki/Using_inheritance

So if I understand correctly, "actor X : Y" means that I am creating a new actor X taking the basic traits of actor Y and they both can coexist, while "actor X : Y replaces Y" means the same but now Y doesn't exist and X takes its place?

Actor X : Y basically means you're creating a copy of the actor Y under a different name without having to copy-paste things, you only need to change what you want to change, you can then replace actor Y if you want.

But can actors X and Y coexist in the same game? For example

Actor BluePistol : Pistol

and I can place both BluePistol and Pistol on the map and treat it as separate weapons?

Thank you, that's what I've been looking for.