Can I feed a sub panel off my 200A service box? Or would I need to run wire from the inside breaker box?
I don't think that's a 125 amp wire. My bet is that's an 8 gauge wire feeding that whole panel, so 50 amps per leg. I wouldn't bank on it being more than that.

Whether or not you can add more is another question. You've got a 20 amp 240v breaker and a 20 amp 120v breaker. You could add another 20A to the one leg with only 20A (the unused slot below the single 20A) and be OK. You just can't exceed 50A on any given leg.