Oh but doesn't "all right" actually mean that everything is good? "- How are you? - All right" would be "Як справи? - Добре".
"Так собі" really means a strong "Meh". I would accept "Meh" as an answer :)
I would consider alright a little short of positive myself, I think, but still more positive than so-so. I'd probably say alright is often close to "can't complain, but nothing to write home about" (though sometimes it could be enough to smile about) while so-so is more like "could be better, but not worth getting upset about"...
It really depends on the context and intonation. However, if you check a corpus of current English, or just listen to people enough, you'll find that no one says "So so." in response to "How are you?". It's not in common use anymore. It would be an acceptable answer, sure. It just wouldn't occur to a lot of native speakers of English, and so it shouldn't be the only possible answer here.
"So so" is common in the United States for when things are not so good, but not too bad.. all right and OK are meaning better than that.
It is an expression I use all the time, and I certainly hear it from other people.
How are you? - So-so.
How was the dinner? - So-so.
Not so good, but not a total disaster either