"So-so" is normal as a response to a question about how things are going. It means things are just okay -- not great, but not really bad. "Fifty-fifty" is also common, but not in this context. We say "fifty-fifty" to describe something that is divided exactly in half, or contains an equal amount of two things. For example, if two people each pay half the cost of a meal, we say they are splitting the bill "fifty-fifty." "Fifty-fifty" is mostly used for talking about money or quantities of things, but you might also occasionally hear something like: A: "Were most of the people at the party vegetarian?" B: "I guess it was about fifty-fifty." ... meaning that about half of the people were vegetarian, and half of them were not. I've never heard anyone say "fifty-fifty" to mean "just okay" or "so-so." (If I asked someone how they were doing and they said "fifty-fifty," I would probably assume that they hadn't heard my question correctly.)

"So-so" is never used in English. You can say "kind of" or "sort of" for this. :)

I'm good I'm fine I'm OK I'm alright These tend to be the common response to how are you.

Both expressions are common among natives, as I know.