I'm quite familiar with -te hoshii, and I have to disagree with your last post.
You can't know what someone is thinking, or what they want. Therefore, you have to use hoshigaru. That's the standard.
Your example equating hoshii with moraitai fails. Moraitai should also not be used for third person desires. Moraitagaru is used.
You can find violations of just about *every* grammatical rule in every language once you start getting informal (even some fundamental ones like use of the copula.)
He did it. -> He done did it.
Are you hungry? -> You hungry?
Does he want to go? -> He wanna go?
Had you mentioned in the original post that it was anime slang, I wouldn't have corrected you; I don't watch enough anime (or any at all) to be acquainted with these things.
At any rate, native speakers can *sometimes* get away with this without sounding uneducated. But foreign speakers usually don't have this luxury. If you speak incorrectly, most people will naturally assume it is because you have learned the language incorrectly. Therefore, you should stick with using the -garu form.