"-ee" and "-er" word endings

There are a few examples of pairs of words ending with -ee/-er like employee and employer or advisee and adviser. What I was curious about is if there was any rule that would describe the relationship of the objects in a pair like this and situations when it's appropriate to create a counterpart for a given word.

I'll give you an example. It's relatively common in the computer programming world to see the word dragee, which describes an object that is being dragged with a mouse. I understand that this is a relatively new word and could not be found in any dictionary (I've tried). Is that acceptable to make up words like this one or is it just bad English?

Actually dragee is in the dictionary: it's a fruit or nut wrapped in sugar (a peanut M&M is a dragee). But anyway!

I would say that if there are no existing words that fit the purpose (as in the case of a dragged object in computing), and the word 'sounds right', then there is no problem with neologising. After all, if no new words came along the language would never evolve.

In general the suffix "-ee" is productive, and usually has the meaning of "person to which xxx is done" - I find "dragee" a little strange because it is not a person.

But beware: there are a few words where the "-ee" denotes the person who does rather than the person who is done to. A prominent example is "attendee", but also "returnee". I think this use arises only where there isn't a prominent object for the verb, because then that meaning would compete for xxx-ee. ("Attend" is transitive, and has as its object the meeting or event; but there is no need for a word "attendee" to mean the meeting or event, which is why I think it has come into use for a person attending.)

From a linguistics point of view this observation is interesting because the words that take "-ee" seem to be grammatical patients - those who undergo an experience as opposed to those who perform actions. English has a nominative-accusative system, in which this role is expressed by different grammatical constructs for different verbs; but in ergative languages, this is generally the role marked by the absolutive case.

You can use the -er/-ee combination on pretty much any verb, as long as it's easily understood.

The example dragee is perhaps not very well suited for this form, as it can be confused with an existing word, and because the result isn't so clearly understood as drager (dragger?) isn't a well know usage either.

My favorite usage of this is in a conversion from Buffy. The -ee suffix is quite elegantly used to express "You are the vampire, and I am going to kill you just like I have killed all the other vampires. That's what I do." in a single word:

Vampire: -- Slayer!

Buffy: -- Slayee!