suffixes - Is ～がる suffix limited to specific adjectives only?

I noticed that there are some adjectives that have ～がる suffix to make them into verbs. Some examples from WWWJDIC:

ほしがる
こわがる
いたがる
くるしがる
さびしがる
うれしがる

Apparently ～たい form also can take the ～がる suffix to become ～たがる suffix:

いきたがる

Rikaichan popup explains the ～がる as:

to feel (on adj-stem to represent third party's apparent emotion); to behave as if one were

So I thought that I could append this suffix to any adjectives to add those connotations. However, it seems that not all adjectives and ～たい have the suffix. WWWJDIC does not return any exact matches for the following words:

やさしがる (to behave as if one were nice?)
きたがる (to feel that one wants to come? - Since いきたがる exists so logically きたがる should exist too?)
ねむがる (to behave as if one were sleepy?)

My question is, as in the title, is ～がる suffix limited to specific adjectives only? How do we know which adjectives can have the suffix and which adjectives can't?

Answer

As explained by the Rikaichan popup you reference in your question, ～がる is a suffix for representing a third party's apparent emotion. As such, you cannot use it with やさしい or ねむい as these are not adjectives which describe emotions.

As it was explained to me, you cannot generally know another person's emotions or thoughts, and so when speaking of them, you use either ～がる (e.g. うれしがる) or ～そう (e.g. うれしそう). To some extent, this is true even in English -- "John seems sad today" seems more generally applicable than "John is sad today" (without John having said so himself).

To answer your question: you use this for adjectives (including ～たい) which represent emotional or psychological states. It's very difficult to provide an exhaustive list of such adjectives.